Britain's announcement comes days after France's national carrier canceled six flights due to terror fears. U.S. officials told French authorities that members of the al Qaeda terrorist network might try to board an Air France flight between Paris and Los Angeles.
The announcement also comes one day after an Italian newspaper on Saturday quoted Prime Minister Silvio Berlusconi as saying the Vatican had been a potential target of an attack with a hijacked airplane. Berlusconi quickly backed away from the comments.
The United States is currently on its second-highest terror alert level — orange. U.S. officials said they believe terrorists are interested in an attack using hijacked commercial planes.
In a related development, CBS News' Larry Miller says a London newspaper reports Saudi Arabian authorities have arrested two men preparing to fly two light aircraft loaded with explosives into a packed a British Airways passenger jet.
The Mail On Sunday reports the men were caught near the Riyadh Airport in the last few weeks. The opposition Conservative spokesman for Homeland Security, Patrick Mercer, said he was told of the plot by an "unimpeachable source" who also claims the Saudis have covered it up and withheld information.
Saudi Arabia has denied the report. British Airways and the British
Foreign Office say they are unaware of the incident.
The British Department for Transport did not specify when or where the marshals would travel.
"Security is kept constantly under review and it is essential that we take all reasonable steps to deter terrorist activities," Transport Secretary Alistair Darling said in a statement.
"Our aviation security program works on many levels, with measures for all stages of the process, from check-in through to the flight itself. Not all of the measures are obvious and a lot goes on behind the scenes."
The British government said earlier this month it was considering putting undercover armed marshals on some flights to prevent terrorist hijackings.
A government-commissioned report on airline safety recommended then that the sky marshals be introduced first on trans-Atlantic flights. If that proved successful, the program should be extended to most flights, said the report by former Conservative Party lawmaker Sir John Wheeler.
Darling's statement, released jointly with Home Secretary David Blunkett, said the marshals would be deployed "where appropriate" but gave no specifics.
Blunkett, Britain's top law enforcement official, cited the orange alert but said Britons should not be afraid to fly.
"What we are proposing is a proportionate and appropriate level of response at a time when the threat to both our countries and around the world remains real and serious," he said.
"I can assure the traveling public that if we believed it was not safe for them to travel or fly we would say so. What we are proposing are some sensible additional security measures."