A British minister said specific information led to the grounding of several British Airways flights this week and warned more cancellations might be needed to prevent another September 11-style attack.
Transport Secretary Alistair Darling said British security services had received information that supported US intelligence.
British defence analyst Mr Paul Beaver
"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 yesterday 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 today the airline planned to operate Flight 223 to Washington, and the plane was due to take off at 16.00 GMT, 55 minutes after its scheduled departure. "The flight is delayed due to increased security measures," she said.
Europe's biggest airline also scrapped today's flight from tomorrow. 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." 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 December 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 on Wednesday and Thursday.