Britain's transport minister said Monday that the vast majority of flights are going ahead as normal despite the cancelation of a London to Washington service twice last week because of security fears.

"The decision to cancel the flight is something that is only taken in exceptional circumstances," Transport Secretary Alistair Darling (searchtold British Broadcasting Corp. radio.

He said such a decision was only taken "when we have information that leads us to the conclusion that that is the only thing that is safe to do."

Hundreds of passengers were stranded when British Airways (searchcanceled flight 223 from Heathrow to Washington's Dulles airport on Thursday and Friday on government advice.

Neither the airline nor British officials would provide details of the threat, although analysts and media reports suggested authorities had received intelligence of a possible airborne attack by Al Qaeda (search)-linked terrorists.

Flight 223 was allowed to take off Saturday and Sunday, but was delayed for more than three hours both days by thorough security screening. British Airways canceled Saturday's flight to Riyadh, Saudi Arabia, citing a government security warning.

"The vast majority of flights to the United States and to other parts of the world have been going ahead as normal," Darling said. "We're trying to strike a balance between increased security at airports, on aircraft themselves, ensuring at the same time that people can go about their day-to-day business."

In response to U.S. security demands, the European Union agreed last month to share airline passenger lists for all U.S.-bound flights with American officials. U.S. officials also have sought to get armed "sky marshals" deployed on some flights.

A dozen flights to and from the United States have been canceled in the two weeks since the Bush administration raised the national terrorism alert to orange, its second-highest level, saying there was intelligence terrorists could be planning large-scale attacks over the holiday season.