The BBC's transport correspondent Tom Symonds explains some of the issues around the introduction of armed air marshals to Britain's transatlantic flights.
The air marshals will only be on certain flights to the US
Q: What are the government's precise proposals for air marshals?
They are shrouded in security secrecy, but the government is not planning to use armed guards on planes routinely.
Rather, they will be used "where appropriate". That appears to mean when there is a threat against British airlines.
The American authorities are demanding precisely this level of security - air marshals must travel on planes entering US airspace which are subject to security concerns.
The government has confirmed pilots will be told they will be on board, and crews will be briefed.
Q: Why are UK pilots opposed to the idea?
For various reasons. They are worried that a firearm carried by a sky marshal could be used against the aircraft or its crew.
But most of all they believe that if there is a level of threat that requires sky marshals to be present, the aircraft should not take off at all.
If the government was to change its policy, and use armed guards routinely, or randomly, on flights where there is no specific security concern, pilots and airlines would have to think carefully about whether they would accept this.
The British Airline Pilots Association (Balpa) wants an agreement that the pilot, as the person ultimately responsible for the safety of a flight, would have the final say about whether it takes off or not.
Q: How many nations use air marshals - and what is their record ?
A minority of countries have introduced them. Israel and America have led the way.
The Israeli carrier El Al has used sky marshals without any major problems for three decades.
In the 1970s a team managed to overpower hijackers on a flight from Amsterdam to New York by warning the pilot, who put the plane into a steep dive, throwing the attackers off their feet.
Last year El Al guards arrested a man with a knife who appeared to be storming a cockpit.
Also last year, two hijackers on an Ethiopian airline flight were shot.
America has had trouble recruiting marshals with the right blend of confidence, and restraint, but the policy is generally accepted in the US.
Q: Are any guns safe on aircraft?
Strictly speaking, no. There will always be the risk of injury, but the level of risk is thought to be manageable.
There is the potential for gunshots to penetrate the cabin walls, but this is unlikely to lead to serious cabin decompression.
Unlike Hollywood's more lurid movies, it is not likely passengers would be sucked out of a plane to their deaths.
Even a missing window can be compensated for by the plane's air supplies, and pilots are trained to descend should cabin pressure be affected.
Aircraft must also carry enough fuel to divert to a nearby airport.
There is the risk that gunfire will hit critical systems, but air marshals are trained to watch the background behind their target.
They also use what is called 'pre-fragmented ammunition' which is designed to break apart on impact, not pass through the body.
Nevertheless, a gunfight on a passenger airliner is to be avoided at all cost.
Q: What is the consensus among security experts on air marshals?
In general that there is nothing wrong with them in principle, but that other security measures are more important.
Security is a chain stretching from the point where passengers arrive at the airport, to the point where they leave at their destination.
Stopping a potential terrorist before they get on the plane is the priority, though many experts believe air marshals are an important last line of defence.
There are concerns that Britain's deployment of air marshals is rather cosmetic and motivated by a desire to please the Americans, more than security reasons.
Q: Are any other safety measures being considered?
Not only considered but actively pursued. The Americans and British are now using "watchlists" of passenger names that might pose a security threat.
The US authorities are fingerprinting and photographing some passengers. Armed police are patrolling the big British airports.
Other types of threats are being examined - British Airways is considering fitting anti-missile systems to its planes, to counter against rocket attacks as they take off and land.