British Airways said flight 223 from Heathrow to Washington's Dulles Airport would not fly this coming Sunday. Monday's Flight 263 from London to Riyadh, Saudi Arabia also was scrapped.
The airline said the decision "follows government advice to cancel those flights for security reasons."
Flight 223 has been delayed or canceled eight previous times this year because of U.S. security alerts. Saudi flights also have been canceled several times.
Officials in Britain have refused to say what intelligence prompted them to advise cancellation of the BA flights, but U.S. authorities have in the past spoken of a "specific and credible" terrorist threat to international flights.
Disruption to flight 223 began Dec. 31, when the plane was kept on the tarmac at Dulles for several hours after landing, while U.S. authorities questioned passengers and crew.
BA scratched the same service the following two days and several times after that, most recently on Feb 2. When the flight has gone ahead, it has sometimes been delayed for hours for stringent checks on passengers. The carrier's two other daily flights to Washington have not been affected.
Last August, BA suspended service to Saudi Arabia, where the British government warns of "a continuing threat of terrorism," after local authorities broke up a cell that reportedly was plotting an attack on a British plane. Service was restored in September.
Several Paris-based Air France flights to the United States and a Continental Airlines Glasgow-to-Los Angeles flight also have been canceled in the past few weeks.
Air France said Thursday that its flights were running normally and there had been no cancelations for security reasons.
Some European officials have questioned the need for the flight disruptions, and Britain's pilots' union has expressed concern over the "erratic" nature of the intelligence leading to the cancelations.