The decision to put air marshals on some UK flights as a "last line of defence" against terrorist attacks follows years of security improvements for air travel.
By Duncan Walker
BBC News Online
More police are on duty at UK airports than ever before
Changes brought in following 11 September mean UK airports boast more armed police than ever before and stringent security checks using the latest technology.
But while aviation experts say it has never been safer to fly, they warn that there is still more to do if future attacks by highly organised terrorists are to be deterred.
Areas of concern include weaknesses in security checks on airport staff and airlines' lack of ability to fully share the information they have on passengers.
Carolyn Evans, head of flight safety for the British Association of Airline Pilots (Balpa), which believes armed air marshals could endanger passengers, said: "We are always aware that there can be improvements."
Security consultant Mike Bluestone warned: "You can never rule out the possibility that someone can get on a plane having been through all the security checks and still have some devious plan in hand."
Heathrow, Britain's busiest airport, is among those to have improved security since 11 September.
A spokesman for its owners, the British Airports Authority, said several hundred more security staff had been employed and that there is now 100% screening of all hold baggage.
"Efforts and resources on security have intensified...and security has remained at the forefront of our mind," he said.
It is a message backed by Jenny Beechener, editor of specialist publication Jane's Airport Review.
She highlighted better baggage screening in the US, and UK trials of machines capable of detecting traces of explosives as examples of security improvements in place long before the introduction of air marshals.
Ms Beechener said no measure was foolproof.
But she added: "The aviation sector is not a soft target...it's high profile, but it's becoming very difficult to penetrate."
Their optimism was not shared by Mr Bluestone, managing director of security consultants the BSB Group, who pointed out that tabloid reporters have repeatedly been able to board planes with weapons.
He said no system could be foolproof and that air marshals were therefore a welcome new deterrent.
"If they're properly trained they can be an asset not just in being an armed response.
"They're pro-active, looking for problems before they occur."
Lapses in security checks for staff like cleaners and maintenance workers were of particular concern to Mr Bluestone.
He said: "If you are a sophisticated terrorist - and al-Qaeda are sophisticated - they plan over a long period of time and they plan well.
"It's logical for them to get people inside the airport."
Balpa, which has threatened a pilots' strike over the use of armed marshals on planes, said it wants to see an international system for the sharing of passenger details.
It believes known passengers could be dealt with quickly, while more attention is given to those who are unfamiliar or are causing concern.
But, Ms Evans said, financing remains an issue when it comes to such sophisticated systems.
She said: "We do see the 'draggy feet brigade' as soon as the question of money comes into it."