Transport Secretary Alistair Darling said British intelligence services had received information that supported that picked up by the United States.

"The threat that we now face is likely to endure for many years," he told BBC Radio. "We are dealing with a different order of magnitude of threat."

"There may occasionally, from time to time, be the need to ground a particular flight... but the vast majority of people will be able to fly in the normal way."

Intelligence information led to the cancellation of BA's Flight 223 from London to Washington Friday for the second day in a row. The same flight was held outside Washington on Wednesday after fighter jets escorted it in.

Passengers were questioned, but no one was arrested.

A spokeswoman for BA said Saturday the airline planned to operate Flight 223 to Washington at 1505 GMT, but warned passengers to check times. "We are operating as normal, but that may change," she said.

Europe's biggest airline also scrapped Saturday's flight from London to Riyadh for security reasons, and its return on Sunday. A decision was pending on Monday's flight.

Asked whether British authorities had information about specific threats, Darling said: "Yes we do. And the reason that different flights are either grounded or there is increased security varies from time to time."

A British anti-terror police source told Reuters the British Airways security alerts were purely intelligence-based, and no arrests had been made or materials found.

The source added security at Heathrow, already at a high stage of alert, had not been stepped up further.

The BA groundings bring the number of cancellations of US-bound international flights to seven since Christmas Eve.

Three Air France Los Angeles-Paris flights were grounded on Dec. 24 and 25, and Air France's New York-Paris flight was diverted to Newfoundland on New Year's Day.

Aeromexico Flight 490 from Mexico City to Los Angeles was grounded Wednesday and Thursday.

Worried about the possibility of another Sept. 11-style attack, Washington is sharing more intelligence with other nations, has asked foreign airlines to put armed marshals on some flights and sent fighter jets to tail some incoming planes.

US officials have said they are uncertain whether they have yet succeeded in foiling a specific plot.

Security experts said governments saw acting on flimsy information as preferable to being blamed after an attack.

"I think you have to understand the position governments are in politically. If, God forbid, something terrible happens and then it emerges they had information of a risk but did not do anything, they would be totally exposed politically," said Kevin Rosser, terrorism analyst at London-based Control Risks Group.