Portugal has joined Denmark and Sweden in rejecting US calls to post armed guards on international flights.
The Portuguese civil aviation authority said that putting loaded guns aboard an aircraft could endanger it.
After meeting the transport secretary Alastair Darling, the UK pilot's union BALPA said there's, "some way to go" before a proper protocol on sky marshals is in place.
The union had previously urged pilots to refuse to fly with marshals on board, but because the US has insisted they are deployed on some flights it wants to agree rules on the way they work.
The US decision comes amid renewed fears that terrorists may be trying to use aircraft to stage a new attack.
Will this measure make you feel safer on flights or should the government be improving security on the ground instead? Send us your comments.
This debate is now closed. Read your comments below.
The following comments reflect the balance of opinion we have received so far:
Some here are having a field day with hypothetical scenarios. Yet some are more plausible than others. The challenge facing all countries is to identify those and respond effectively. That is the purpose of the air marshals program and the fingerprint requirements. Imperfect and incomplete solutions, but reasonable. It's far easier to speculate and criticize, isn't it?
It is all a matter of intent. Select the good guys, arm them (and if necessary and they are right for the job, the air crew too). I believe this policy has worked for those operating El Al, for decades, and their hijackings seem very rare.
Chaz Forsyth, Dunedin, New Zealand
I think you just keep the cockpit locked and no armed police on the planes. I've seen people have said that that it's worked with El Al but in reality it hasn't stopped the terrorists in Israel they just blow themselves up on buses or in shopping centres instead. They haven't solved the problem just moved it on.
I can see no problem with armed air guards on both International and domestic flights. It is time that people realise that there is a real terrorism threat out there and it is the role of respective governments to protect their own citizens and those who are travelling within our country. I am heading to the US next week and would feel much safer if I knew that there were armed guards on my flight.
Reilly, Melbourne, Australia
No. Absolutely not. Step up the security on the ground. In the last four years I have passed through America four times, the security at Los Angeles and Miami airports was virtually non-existent. I saw my baggage passing through without check and the people checking passports and travel documents were about as interested in their job as anyone would be in watching paint dry, none.
Bob Beadman, Hong Kong
Call me paranoid, but the terrorists that this plan is trying to deter are the ones that are in it for the long haul. They will thing nothing of working to become an Air Marshals. I would be interested to know exactly how many UK airlines have been hijacked as well, as I believe we have very good security.
They are no real use. Better to have doors to the cockpit which can't be forced under any circumstances. If terrorists have a bomb, marshals won't be any good anyway. There are so many ways terrorists can cause death and fear, not just by hijacking planes.
Chris C, London, UK
Very good idea - pilots are supposed to fly planes, not overpower terrorists so let's have an expert on board my next flight please.
P.S. ref "no more queuing for aircraft loos" - hopefully some bright spark will install deli-style number machines on the loo door. We take a number, sit down again and wait for our number to flash up on the overhead displays, already in place for seat belt warnings etc.
Joss Barnard, Ramsgate, Kent
What if there are enough hijackers on board the aircraft that one group deliberately activates the air marshals by attempting to hijack the flight deck at which point the second group who had not yet been identified then can attack the marshals and if successful they acquire their weapons. If the hijackers are going to turn the aircraft into a flying bomb they won't mind loosing a few in order to capture the weapons and the aircraft.
If I was a hijacker, the fact that armed marshals were on planes, would mean that I don't have to try and smuggle a weapon onto a plane, there is already one there, all I need do is overpower the marshal. Loads easier.
Some of these concerns over air marshals being disarmed are comical. If a terrorist bent on using the aircraft as a weapon were to successfully hijack it, it wouldn't matter what weapon he/she was wielding, be it box cutters, the threat of a bomb, or an air marshal's firearm. At that point, the question is whether the passengers and crew can successfully retake the plane or whether it needs to be shot down. I'd prefer highly-trained air marshals as a third option.
Many of the comments from US residents seem to ignore the fact that the prime requirement for using an aircraft as a flying bomb is that it is full of fuel. This is why the domestic flights used in 9/11 were so devastating in their effect. Transatlantic flights result in aircraft arriving in the USA with nearly empty fuel tanks. Not a good idea for terrorists. So much easier to hijack a domestic flight, given the clear inefficiency of US domestic security.
David, Winchester, England
To David, Winchester, England: No, we Americans are not ignoring that fact. It has been talked about in our media quite a lot. However, the terrorists are now finding it hard to board a US domestic plane because of our security measures. So, they may settle for an international flight. Just because a plane isn't completely full of fuel doesn't mean it can't do damage if it is flown into a target. Something is better than nothing as far as these terrorists are concerned.
James, Jamestown, VA USA
We hear these guards are "Ex-police" or "ex-army". Why not current police? Who employs them and who trains them? Also the name is fairly inappropriate given that "Air Marshal" is a senior rank in the RAF.
Dan Campion, Manchester, UK
To Dan Campion, Manchester, UK: The term Marshal comes from America where it means a policeman with sole authority to act in a situation. There are plenty of US Marshals who have nationwide jurisdiction and are used for special law enforcement purposes. If the term is being used in the UK it has simply been adopted.
I am sick and tired of the UK following the "trend" of things American! The USA should take a long hard look at themselves and solve their own problems with the world but without getting the rest of us involved. Half the problems in the world are created by the US not solved by them. Personally I would not fly on a plane if I knew it had an armed person on board, no matter who they were or what their training and if they were American, I would keep as far away as possible.
Archangel, Yorks, UK
As someone who does not fly under any circumstances, may I give a disinterested opinion? Are the Americans not again attempting to treat the symptoms rather than treat the root cause? If the US spent half the energy devoted to belated air travel 'security' measures on trying to obtain a just peace in the Middle East and curbing their own modern version of gunboat diplomacy, these absurd measures would not be required. This is a typical knee-jerk reaction; I would heed the advice of the airline pilots.
Charles, Bolton UK
I think it is a good idea to have someone on board who is able to deal with any situation that may occur (terrorism, air rage etc). They don't have to be armed with bullets they could use tranquillisers but certainly should be well trained in un-armed combat. It seems sensible to have someone else other than the crew as they have chosen this particular career as flight attendants/pilots not police/armed forces. It should not be up to the captain to have to stop flying the plane to deal with these situations. In this day and age surely more things should be done to ensure our safety when flying.
Rossana, Newcastle upon Tyne, UK
I think having a gun on board a plane is potentially lethal. It won't take much for a marshal to be overpowered and for the gun to be used against the people it was meant to protect. Also there is no guarantee the bullet won't puncture the aircraft resulting in depressurisation and crash.
Michele Brown, Orpington Kent. UK
It's simple really - if a flight is deemed to be at risk from a terrorist threat then it should not leave the ground. Just because America is doing it does not mean we need to follow their gung-ho attitude.
Frank Mills, Milton Keynes
Air marshals are important for their deterrent effect, nothing more. No one wants to see gunfights occurring in airplanes and I strongly believe that no one will. In fact air marshal systems have existed on US and Israeli flights for years and there has yet to be an incident where a flight was brought down through their actions. What the air marshal system does is place doubts in the minds of terrorist planners that they could successful conduct a hi-jacking of a flight if there is an armed guard on board. Terrorists are looking for slam dunks, not attacks that could be easily foiled.
I love the idea that we have been carefully keeping weapons off planes for so long just so the USA can start giving an in-flight direct supply to any terrorist who beat up a sky marshal. Well-done USA! A very well though out plan.
HB, Sheffield UK
Air Marshals are a logical extension of the concept of 'Transport Police'. Given the nature of the terrorist threat they obviously need to be armed.
I will certainly feel safer flying when we have Air Marshals on all flights. However, I just hope they're significantly better trained, better equipped and better supported than our 'regular' Police, given the UK's poor track record in equipping, training and supporting all but the 'elite' units.
To those who argue that the hijackers will just overpower the air marshal and take his gun - how may times has this happened to El Al? If the aircraft is not hijacked the Air Marshal poses no threat to anyone. If it is hijacked, how do you expect to stop the hijackers and save the passengers without a trained gunman who has access to weapons?
Steve Burrows, Hebden Bridge, UK
The air marshal concept does not bother me that much. What bothers me greatly is the arrogant way that measure is introduced. Before any (non urgent) measure that affects a lot of people is applied the people mostly affected (pilots, air line companies, travellers etc.) should be consulted. After all they might offer ideas that would make the measure more affective. Also, whenever armed security officers are deployed the public must be aware of the guidelines and rules of engagement they operate under, otherwise we will continue to have members of these security forces acting with impunity to prosecution even when it is proven that they operated beyond the law.
Aris Bonsalendis, Athens/Greece
The problem is too complex. One would think that air-marshals can be the answer but I doubt it. As long as you are telling us that air-marshals have been introduced to anticipate any terrorist activities the modus operandi of terrorists is going to change. Yes psychologically there is sense of safety but in reality the danger can come in different forms.
Eddie Sibiya, Durban/South Africa
Obviously ground screening is the main line of defence and in the ideal world the terrorists should never get on board. However we live in the real world and this isn't and never will be foolproof. When (not if) a terrorist gets on board, there must be a defence of some sort. Air marshals would be trained specifically for this job. There is not really a credible second option once the main defence has been breached.
David, The Netherlands (UK expat)
Many countries have air-marshals on planes such as Russia, India, Israel and many others. I agree with all the other posters that air-marshals only give a false sence of security where the real work has to be done at airports, however, a professionaly trained anti-terrorist expert on a plane is still beneficial.
Mike S, Chicago, US
It would mean that the terrorists don't have to smuggle a gun aboard or use something like a box-cutter to hijack the plane. Just let the marshal bring the gun aboard legitimately and work out a way to get it off him. Then start shooting passengers and crew so the pilots are forced to open the cockpit.
Ettenuahs, Swindon, UK
Prior security is important rather appointing marshals.
Pradeep Shukla, Chicago, USA
I've read comments that air marshals pose threats to flights. So, why don't we make it public which flights have air marshals and which ones do not? Then we'll see which flights have more passengers and which flights are more likely to be hijacked. And, besides, they can always make guns which won't fire in the hands of anyone other than the person who's supposed to have it.
Jon Jannicola, NYC, USA
To many who flew domestically in the US prior to Sep. 2001 the fact that 9/11 could happen wasn't exactly shocking. The US was slack about its security procedures at airports for many years, in the main due to the fact that Americans demanded that flying remain cheap. Now because they screwed up, they expect the rest of the world to accept their 'security is a loaded gun' policy. One day the US will understand that it can learn something from the rest of the world but sadly I don't think it will happen in my lifetime.
Ben, London, UK
It appears from the posts here that Europeans are still under the delusion that if a hijacking occurs, they won't die. Wrong. Wake up. It's a different world out there. Since 9/11 one can assume that if a plane is hijacked, everyone on board is going to die, unless there is an Air Marshal there to prevent it from happening. Gone are the days when planes were hijacked for ransom. Terrorists will from now on take planes with the intent to use them as weapons against a large target. I for one, would like to know that if that happens, there is at least one good guy on the plane that might be able to stop them.
J. McKenzie, Flagstaff, AZ USA
I don't think, that one or 2 air marshals could secure a flight with 5 or more terrorists on board. Increasing security on the ground will be more effective in preventing terrorist-attacks like 9/11th.
Benjamin Satheesh, Germany
This is crazy; now a hijacker doesn't need to risk getting weapons onboard as they are already there. With a sufficient number of hijackers a staged incident would lead to the air marshal being identified and then overpowered. Result, a hijacked plane.
Terry, Vancouver, Canada
All I have heard here is complaining and not once someone come up with another solution. I think Air Marshals are a good idea.
Jan, Seattle, USA
I have been flying on domestic American flights all of my life. I feel safer knowing that the air marshals, who have been serving on such flights all along, are there to protect me should any hostility occur. The bottom line is that this is nothing that we have not had on our own airliners for years. Now that it is so apparent that airliners make effective missiles, we are asking that the same precautions be taken on airliners that fly to the USA.
Tom, Denver, Colorado, USA
I don't know what people are getting so excited for. A lot of people are saying they don't like the idea of having weapons on board. The sky marshals would only use their weapons in case of terrorist activity and at no other time. If there were terrorist activity on board and they had to use their weapons, don't you think the terrorists would have even worse weapons than the marshals? In such a case I would then be glad to have them on board. Otherwise you wouldn't (or shouldn't) even know they're there!
Mike, Germany (US expat)
I was just curious to find out what function air marshals would have seeing as every passengers' credentials are checked before they board by two countries. A free flight I guess, how nice! Where do I sign up?
Gabriel Asseily, UK
The concept of air marshals is scary. Guns are not the deterrent we believe them to be. We could be playing into the hands of terrorists. One of these jobs could so unwittingly be given to them. Prevent the terrorists becoming airborne - toughen up ground security.
Sally, Sussex, England
Please correct me if I'm wrong, but the planes involved in the tragic events of 9/11 were all US domestic flights. Why then should the rest of the world be subjected to US security policy when the security breach occurred exclusively in the US?
Nick, Thatcham, Berkshire
Other countries should simply demand armed guards on flights to their countries as well. After all, the Bush administration keeps telling us that terrorists don't discriminate on the basis of nationality. Besides, it's not fair that foreign carriers, travellers and inevitably taxpayers pay for a measure primarily designed to protect American citizens, while their own security might be at risk.
Paul, Toronto, Canada
It's a curious thing to see the posts on this board. Americans are mostly in favour of sky marshals while Europeans trot out their anti-gun sentiments and stories of mishaps that could happen. The bottom line is that 9/11 happened in the US and therefore we feel the danger more keenly. We know for a fact that Al Qaeda would like to repeat 9/11. Since other countries don't feel they are the target, they feel no need to take these measures. Fine. However, if a plane is flying over or to the US from another country, they should comply with our rules, for the safety of our citizens on the ground as well as the passengers on the plane.
Sherry, Destin, FL USA
Armed guards on an aircraft would make me feel less safe. Police are armed in the US now, and it hasn't stopped armed crime, therefore armed guards on a aircraft will not stamp out hijacking or other terrorist activity. I believe that it would raise the stakes when a hijacking takes place, possibly resulting in greater loss of life.
The best way to prevent these attacks is to improve security at the airports where the planes are loaded in the first place.
Chris Farrow, Spain
I find it interesting that the pilot's union is opposed to this. Pilots have the safety of a secure cockpit. At the first sign of trouble, they can and must lock themselves in that cockpit. But what about everyone else on the aircraft? Surely, terrorists will harm passengers and crew in an attempt to get the pilots to open the door. As a passenger, I could care less what someone who can lock themselves in a bullet-proof room has to say about sky marshals, who are there to protect me. I don't want armed guards on all flights but those that fall into certain risk profiles should employ them.
Jim , NJ, USA
Two points: How much will this add to the price of my ticket and who will get bumped if the flight is over subscribed? Bottom line is me...
Rob Jones, Chesham, Bucks
Do we really have a major issue with planes being hijacked? Fair enough two and a half years ago the world experienced hijacking being taken to a new level, but how many planes have been hijacked since? Not many and with no air marshals in sight. I think that the efforts should be placed on increasing security on the ground. Air Marshals will be a waste of time and money because they will not be used effectively.
Marc, London, UK
To those who complain about the "unilateral" requirement that foreign airlines employ air marshals, you seem to forget that this applies only to flights entering American airspace. The US government has the authority to regulate American airspace. If certain airlines refuse to abide by the regulations, they can simply stop flying into American airspace. I'd bet, though, economics will trump all these pacifist high ideals.
John S, USA
Air marshals are simply a big distraction and a big waste of time and resources. It will do no good because the terrorists would strike when they feel the time is ripe. The FBI and the CIA should go to sleep and stop giving people fake intelligence reports. Where were they when the two world trade centres were brought down in full view of the world press.
Diogu Emeka, Nigeria
After serving as a pilot for a major US airline for thirty years, I am totally opposed to the so called sky marshals. During my career, I have been on a number of flights where difficult incidents have occurred, sometimes due to disturbed passengers. On every occasion the situation has been handled without excessive drama. An armed individual could well escalate the situation out of control. As soon as an armed individual is on board, effectively the captain is no longer in command.
As long as we have bullet proof cockpit doors and bulkheads, and burst proof locks we most certainly do no need weapons in the cabin. There is also the very real danger of an innocent passenger becoming aware of the weapon and attempting to disarm and kill the marshal. In fact some years ago a similar situation has already occurred, fortunately without injury.
Captain K. Owen, Hanover, USA