Armed undercover police have been trained for use on UK passenger flights, Transport Secretary Alistair Darling has announced.

The news came the day after a senior Whitehall source said there was a "high probability" international terrorists would sooner or later launch an attack on the UK.

This new capability has not been developed now in response to any new or specific intelligence Alistair Darling

Mr Darling said the introduction of air marshals followed a decision earlier this year to reinforce in-flight security as part of the ongoing review of aviation security after the 11 September attacks.

The threat to UK aviation remained "a real one", he said, but the new measure - following the example of Israel and Australia - had not been developed "in response to any new or specific intelligence".

Airlines, including British Airways and Virgin Atlantic, have raised concerns about the prospect of guns being carried on board planes.

A warning of a continuing threat from al-Qaeda - possibly targeting planes - was issued at a briefing for reporters on Wednesday.

In a statement on Thursday Mr Darling said: "This further security measure joins others the government has taken to increase security both on the ground at airports and in flight since the attacks in the USA."

Mr Darling said the government was "moving faster than the international community at large" to ensure UK aircraft were fitted with reinforced flight deck doors.

The type of weaponry and the calibre of ammunition used is designed not to penetrate the skin of the aircraft Mike Bluestone Security expert

"Last month we acted to ensure that flight deck doors on foreign aircraft are kept locked, as they have been on UK aircraft since very soon after the US attacks.

"We have also placed strict limits on those able to be on the flight deck of UK aircraft."

The marshals will be used on both domestic and international flights but officials refused to say whether government would insist marshals were used.

A Downing Street spokesman said: "We are not going into details of when and where and how many.

Nations using marshals:

Israel Australia Germany America

"We understand people's reservations but these people will be specially-trained.

"We are not taking a bobby off the beat and putting him on an aeroplane."

And an independent security expert said crucially, their guns would be made especially for use inside a plane.

"The type of weaponry and the calibre of ammunition used is designed not to penetrate the skin of the aircraft, but to actually deal with a human target," said Mike Bluestone, head of BSB Group security consultants.

The idea has prompted concern in the aviation industry.

A British Airways spokesman said: "We are concerned about the presence and use of firearms on board our aircraft.

Airport security has been tightened since 11 September

"However, we are working closely with the government to make sure these plans do not jeopardise on-board safety and to ensure that the correct procedures are in place should deployment be necessary."

UK airline pilots' association Balpa, said the emphasis should be on preventing terrorists from boarding planes in the first place.

Balpa's technical secretary, Carolyn Evans, told BBC Radio 4's World At One: "We do not believe that putting armed police on the aircraft is necessarily the cure.

"The risk of the terrorist disarming the guard and using the weapon himself is a high one to take."

Public alarm?

That was a worry echoed by Gwyneth Dunwoody, chairman of the Commons transport committee.

Conservative shadow transport secretary Tim Collins said the idea should be tried if security services were convinced it was the way forward.

"Many people, however, will be anxious to be reassured that taking guns onto an airliner does not create any additional opportunities for hijackers," added Mr Collins.

Liberal Democrat home affairs spokesman Simon Hughes said using air marshals where there was a specific threat to planes could be justified.

But he warned: "Armed air marshals as a matter of course are not going to add to public confidence, only to public alarm."

'Serious threat'

Government officials believe the most likely form of an terrorist attack is against the transport system or some form of unsophisticated chemical warfare, or using high-explosives in a conventional bomb.

My fear is that terrorists' capability will grow Government source

A government source confirmed that "small numbers" of al-Qaeda terrorists were operating in the UK.

An attack was "not inevitable" but such groups were extremely determined.

"The sensible precaution for the nation is a sustained campaign to improve our national resilience," he added.

Since 11 September, the government has stepped up searches on staff, passengers and their hand and hold baggage, vehicles, cargo and catering, with a particular emphasis on flights going to key destinations such as the USA.

The range of articles not allowed aboard aircraft has been extended and more funding has been channelled into airport policing.