London, Jan. 4 (Reuters): Britain warned today that travellers face years of severe security alerts like one that forced several international flights to be grounded last week amid fears of a September 11-style terror attack.

Transport secretary Alistair Darling said exceptional circumstances and specific information about a possible terror threat led British Airways to cancel two flights from Britain to Washington and one to the Saudi capital Riyadh last week.

“For many years to come, we are going to be living in an age where there is going to be a heightened state of alert. Sometimes it will be quite severe,” he told BBC television.

“Where we have to cancel a flight, the grounds are very clear in our minds and we are justified in taking that decision.”

A BA flight from London, grounded on Thursday and Friday due to security alerts, finally left yesterday and landed in Washington today morning.

Passengers said security had been extremely tight at London’s Heathrow airport before their departure, but the flight itself aboard the jumbo jet was uneventful.

Flights from Mexico and France to the US were also cancelled over the holiday period due to security fears.

Darling declined to comment on the cause of the BA alert.

Britain’s Sunday Times newspaper, citing a senior intelligence source, said security services had word of a plot by the al Qaida again to hijack several jets — including a BA plane — simultaneously and crash them into big US targets, in a re-run of September 11.

The Sunday Telegraph quoted security sources as saying two al Qaida members were at large in Britain and planned to detonate a shoe bomb or similar device in an aircraft lavatory.

“Most people would expect the government to look at all the information they have got and then reach a decision as to what is best to enable the aircraft to fly safely,” he said.