BRITISH Airways has cancelled today's flight from Heathrow to Riyadh, in Saudi Arabia, for security reasons, the airline said last night.

The move follows further cancellations to BA's US flights - brought about because of terror warnings and heightened security alerts - which have left hundreds of passengers stranded.

Flight BA263 was due to leave Heathrow at 1.35pm, arriving in the Saudi capital at 10.55pm local time.

The airline said the return flight BA262, which was due to leave Riyadh tomorrow, has also been cancelled.

BA said it was advised on Wednesday to cancel the flight.

A spokesman said last night: "Following the cancellation of last Wednesday's flight from London Heathrow to Riyadh in Saudi Arabia, British Airways has cancelled tomorrow's flight to Riyadh.

"On Wednesday, the UK Government advised the airline not to operate the flight for security reasons.

"The decision to operate next Monday's flight to Riyadh will be kept under review throughout the weekend."

The move follows the airline's decision earlier yesterday to cancel a flight from London to Washington for the second day running.

About 300 passengers checking in to the fully-booked flight BA223 from Heathrow were told two hours before it was due to take off that the flight was grounded.

It was the third flight on the route to be affected by security concerns. BA223 was grounded on Thursday after the airline received security advice from the Government, and flight BA222, which was due to arrive at Heathrow at 10am today, was also cancelled.

On New Year's Eve, a BA flight was escorted into Washington Dulles International Airport by F16 fighter jets and kept on the runway for three hours to allow security officials to board the plane and question passengers.

Neither the airline nor the Government would say why the latest decision was made, but a security expert said it was based upon "a real and definite threat" from terrorists, probably targeted at the US capital.

Defence analyst Paul Beaver said: "This is certainly unusual. The intelligence is very, very precise - which is why this one flight has been cancelled.

"We have got intelligence, I am told, that there was a plan to take the aircraft and destroy it over Washington or fly it into something. Washington is the definite target."

However, last night's BA225 flight to Washington from London took off at 6.40pm as scheduled, the airline said.

Home Secretary David Blunkett held talks yesterday with US Homeland Security Secretary Tom Ridge to discuss ways to minimise disruption on flights to the US amid the increased security levels.

The Home Office said the talks had been useful, but no specific plans had emerged.

The warnings come as the US steps up security, heightening its alert status to orange, the second highest possible.

An Aeromexico flight from Mexico City to Los Angeles was cancelled after US authorities refused to allow it to land because of security worries.

But Mexican officials found no evidence of a threat on flight 490 and all 40 passengers were allowed to leave on a Mexicana flight.

* Six Air France flights from Paris to Los Angeles were grounded last month because of concerns that terrorists were on board.