Home Secretary David Blunkett and Transport Secretary Alistair Darling said the move was a "responsible and prudent step" in response to the heightened state of alert in the US and would be deployed "where appropriate".

Last week America raised the national alert to code orange, the second-highest level, after reports that a French airliner would be hijacked and used as a missile against a US city.

The plan, which is part of a general tightening of airport and airline security in the wake of the September 11 attacks, is opposed by the British Airline Pilots Association (Balpa).

Balpa general secretary Jim McAuslan said: "We do not want guns on planes. It is dangerous to have guns on planes and there are other things you can do, like investing in security measures on the ground."

He said the union would be advising its members not to fly if they were "not comfortable with arrangements".

British Airways and Virgin Atlantic have declined to comment.

Flights cancelled

The announcement follows a number of security scares involving airports in recent days.

Last Wednesday six Air France flights between Paris and Los Angeles were cancelled amid fears of a terrorist attack.

Today's announcement follows a similar move by the Australian Government two days ago. It said it would place sky marshals on Qantas flights to Singapore.

The Israeli airline El-Al, famous for its strict security, has been using armed marshals for more than 30 years. The German airline Lufthansa also has them on some of its flights.

In a separate warning yesterday, the British Government said it believed terrorists could be in the final stages of planning an attack in Saudi Arabia.

British nationals have already been warned against all but essential travel to the kingdom following attacks in Riyadh in May and November this year.

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