Air passengers' travel plans were thrown into chaos today after another computer failure at an air traffic control centre.

Some flights were cancelled and many others delayed following the problem at the West Drayton centre in London.

Gatwick was not as badly affected as Manchester and Heathrow but suffered some hold-ups.

A Gatwick spokesman said the computer failure had briefly affected flights leaving the airport, but had not caused significant delays.

He said: "We have had no major disruption. If it had been on a heavy weekend then we might have noticed it, but it did not cause problems."

British Airways cancelled two flights from Heathrow - one to Brussels, the other to Frankfurt - and warned passengers to expect delays to other services.

Some Heathrow long-haul departures were delayed for up to two hours while at Manchester passengers faced delays of up to three hours.

A spokesman for Stansted airport said passengers should expect a two-hour delay.

The problem at West Drayton follows a similar computer failure, which led to flight cancellations and delays just before Easter.

Today's difficulty began when a computer which produces electronic flight information failed at 6.05am and was not up and running again until 6.21am.

This meant that the information, which is passed to the new £600 million air traffic control centre at Swanwick in Hampshire, had to be produced by hand.

A spokesman for National Air Traffic Services (Nats) said: "This is a much slower process and has meant that, inevitably, there are delays.

"This is the same computer that failed before Easter. We are investigating what went wrong and we apologise to airlines. We should point out that safety was not compromised."

These incidents are a further setback for the newly-privatised Nats, which received a £30 million bail-out from the Government after running into financial difficulties following September 11.

Richard Everitt, chief executive of Nats, said the system was back to normal by 8.30am. He added: "We are very sorry for the inconvenience caused to passengers and the airlines, and we are going to investigate this very thoroughly this morning."