Armed sky marshals will be placed on some British passenger planes after warnings from the United States about a heightened terror threat, the British government said yesterday.

An official said the move did not suggest there was intelligence about an imminent threat but described it as a "responsible and prudent step".

"It is essential that we take all responsible steps to deter terrorist activities," Transport Secretary Alistair Darling said in a statement.

The marshals will be dressed as normal passengers. They will not be deployed on all flights but used "where appropriate".

"The last few days have seen the United States increase their general threat and security levels," British Home Secretary David Blunkett said in a statement.

The government said last year that it had trained police to perform the role if necessary. Then, a number of airlines and pilots' union BALPA expressed concern about guns being allowed on planes.

Six flights between Paris and Los Angeles were cancelled last week amid fears in Washington of an attack similar to those of September 11, 2001.

In December 2001, Briton Richard Reid tried to ignite an explosive device hidden in his shoe during a flight from Paris to Miami. He was overpowered and later jailed for life.

Sajid Badat, a British Muslim, appeared in a London court earlier this month on charges of having explosives and conspiring with Reid to carry out terror attacks.