Britain said on Saturday a specific threat led to the grounding of several British Airways flights this week and warned that more cancellations might be needed to prevent another 11 September-style attack.
Amid reports members of the Usama bin Ladin's network were plotting another spectacular aviation attack, Transport Secretary Alistair Darling said British security services had information supporting US intelligence on a risk to the cancelled BA flights.
"The threat that we now face is likely to endure for many years," he added in a BBC interview, without giving more details. "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 on Friday for the second day in a row. The same flight was held outside Washington on Wednesday after fighter jets escorted it in.
On Saturday, Flight 223 to Washington took off from London, delayed by three hours because of extra security moves, including passing information to the United States, BA said.
Police with machine guns stood guard at the boarding gate while others with sniffer dogs mingled among passengers. A dozen officers quizzed all passengers before they were allowed on.
"When we take off it will have been searched so thoroughly it will probably be the safest flight in the sky," said British passenger Shirley Newman, 56.
BA, Europe's biggest airline, also scrapped Saturday's flight from London to the Saudi capital, Riyadh, for security reasons, and its return on Sunday. A decision was pending on Monday's flight.