You won't know they are there, but you could be sharing your next flight with an armed guard trained in hijack prevention.

"Sky marshals" are one of the most controversial anti-terror measures to be considered since the September 11 attacks, when four US planes were taken over in one morning.

It raises the prospect of a shoot-out on an airplane Kieran Daly, Air Transport Intelligence

But a number of incidents since, including the attempt by British-born Richard Reid to blow up an airliner with explosives hidden in his shoes, has convinced the government to act despite the concerns of airlines.

Kieran Daly, editor of Air Transport Intelligence, says there is a great deal of nervousness about the plans.

"The whole security system is based on preventing guns or explosives from getting onto an aircraft. Now they would be on an aircraft with the intention that they would be used. It raises the prospect of a shoot-out in a small and confined space," he says.

"The jury is out on whether this is a good thing to do."

There are other dangers too, he adds.

"What happens when there is serious air rage? Is the marshal just going to sit there or threaten someone with a gun? Potential terrorists could start a minor incident in order to identify the marshal. It would take a disciplined person."

Airline concern

David Learmount, of Flight International magazine, says well-trained guards would have a good chance of winning an on-board conflict.

"Sky marshals should be seen as the final line of defence. Using them is an admission that the airport security has failed," he says.

Airlines are concerned about the presence of guns on-board flights, and the danger of scaring already nervous passengers.

Richard Reid admitted trying to blow up an airplane

British Airways and Virgin have voiced reservations, pointing instead to the concerted efforts to improve other aspects of air security, such as more comprehensive airport searches and the fitting of stronger cockpit doors.

But other countries have embraced the idea.

Israel's national carrier El Al has had armed guards on its flights for many years, and they have been used sporadically in the US.

Since the devastating 2001 attacks in New York and Washington, the US has approved plans to increase the use of guards on its flights, although they are always covert.

Australia has also trained a team of experts armed with low velocity firearms that can kill but will not penetrate the fuselage of an aircraft.

We've got to have them too Eric Moody, former BA pilot

Other countries are considering similar moves but the sensitive nature of any deployment means few operational details are ever given. Such secrecy is designed to act as an extra deterrent.

Former BA pilot Eric Moody thinks resistance to armed guards in the UK will ease.

"If other countries are going to have air marshals we've got to have them too. It's like being the only house on the street without a burglar alarm. That's the one that burglars will go to," he says.

But many opponents of armed marshals say biometrics - advanced identification techniques - and new profiling databases are the best ways to prevent terrorism.

Captain Mervyn Granshaw, chairman of the UK airline pilots' association Balpa, says: "We have always supported enhanced security measures, but we believe the emphasis must be on preventing potential terrorists boarding an aircraft in the first place.

"This means having effective counter measures such as passenger profiling."