New Yorker Lauren Richardson celebrates her arrival on Flight 223

The scheduled 1505 GMT departure on Saturday was delayed while all 225 passengers were searched, checked and escorted onto the plane one at a time.

It took off more than three hours late fromm Heathrow and landed at Dulles Airport at around 0210 GMT on Sunday.

It had been feared terrorists posed a "specific threat" against the flight.

Relief

Passengers gave a round of applause when the plane finally landed and they reached the terminal area an hour later.

BBC correspondent Andy Tighe, who was aboard the plane, said the atmosphere had been calm and there were few complaints about the delay.

Many people, myself included, were pulled aside by Special Branch officers just before we got onto the plane and quizzed about why we were going to the United States BBC correspondent Andy Tighe

"A lot of people probably put to the back of their minds the very real fear - that people at senior levels of the American Government must have had - that this plane could have been a target for a 11 September-style attack.

"So relief was the predominant emotion on behalf of most of the people on that flight."

A BA spokeswoman said the plane had been delayed "due to a request from the security authorities in the US to supply them with information about the flight".

FLIGHT SECURITY FEARS Christmas - six Paris to Los Angeles flights cancelled Wednesday - AeroMexico flight from Mexico City to Los Angeles cancelled Wednesday - BA flight 223 shadowed by fighter jets, plane searched and passengers questioned Thursday - BA flight 223 from London to Washington cancelled Thursday - Air France New York flight lands in Canada for baggage check Thursday - BA flight from Washington delayed for extra security checks Friday - BA flight 223 from London to Washington cancelled Saturday - BA flight 263 from London to Riyadh cancelled. Return flight on Sunday also cancelled.

Security fears also grounded BA flight 263 to the Saudi capital Riyadh on Saturday, plus its return on Sunday.

Transport Secretary Alistair Darling said the flights to Washington and Saudi Arabia had been cancelled due to "specific information".

He told BBC Radio 4's Today programme he would not give a "running commentary on specific threats" but the UK Government had access to similar intelligence as the US and acted upon it.

BBC security correspondent Frank Gardner said his sources believed the flight was cancelled on Thursday and Friday because of fears a terrorist suspect was planning to hijack the aircraft and crash it into a target in Washington.

An official with the US homeland security department confirmed there had been "very specific and credible concerns" about the flights.

Passports

However, there were separate reports in the New York Times, quoting an American official, that the BA flights had been grounded because of pilots' reservations about flying with air marshals on board.

And on Sunday, the Observer newspaper said an internal BA memo showed deep opposition among executives to the government initiative.

Police monitored boarding passengers

BA's operations director, Mike Street, had hinted the airline would refuse to fly with air marshals on board because it would mean there was a threat to passengers, the paper said.

The Observer also reports there are fears that terror suspects thought to be planning an attack on BA flights may hold legitimate UK, US or other European passports.

It claims new evidence suggests extremists are trying to recruit suicide bombers from educated classes in the UK, because they can more easily penetrate tight security.

On New Year's Eve flight 223 was kept on the runway for three hours after landing at Washington Dulles International Airport to allow security officials to board and question passengers.

BA is considering this weekend whether to resume flights to Riyadh on Monday.

Mr Darling warned the terrorist threat was likely to continue for the foreseeable future and may inconvenience for air passengers.