Thousands of air passengers faced delays on Wednesday after a technical problem at the air traffic control centre in West Drayton, near London, meant that flights across the UK had to be temporarily halted.

BBC transport correspondent Tom Symonds examines what caused the problem and its implications for passenger safety.

What went wrong?

The computer that failed at West Drayton produces the data about each flight that is passed to the main control centre at Swanwick in Hampshire.

There, it makes up a strip of information relating to each aircraft that controllers use to keep track of traffic.

For 20 minutes the information flow failed, and staff at Swanwick had to write out the details by hand - part of the procedure for dealing with a computer failure.

The problem is that the system becomes less efficient so planes on the ground have to be prevented from taking off so that those arriving can land.

Even when the computer was working again, it had to be tested by gradually increasing the flow of traffic back up to normal levels.

The knock-on effect of this is a series of delays to flights.

How worried should we be about this?

Firstly, this sort of breakdown does not put safety at risk - radar equipment and communications were unaffected, and if safety had been in doubt, all planes would have been grounded.

But it does disrupt the flow of the country's air traffic. The computer has recently been upgraded, so NATS staff will want to find the faults - whether they are in software or procedures - as quickly as possible.

How major are the delays?

Most UK airports are now operating a normal service, but some flights were delayed by up to two hours during the morning.

Earlier there was a complete suspension of flights at Birmingham airport, one hour delays at Manchester and two hours at Heathrow.

Two flights had to be cancelled by British Airways.

Flights from Gatwick Airport were unaffected.

This has happened before, is it likely to happen again?

These sorts of problems were happening 18 months ago but NATS has done a lot of work on its computer systems since then.

Two weeks ago there were delays of up to four hours when the system failed because an operator entered incorrect data.

The question now is whether the same faults are reoccurring or whether this is a series of different faults.

The problem is that the brand new air traffic control centre at Swanwick relies on the flow of data from this computer. A NATS investigation is underway.

Is this a big blow for National Air Traffic Services?

It is the last thing managers want to see happen.

They thought the system was running smoothly after Swanwick went on line in January, only to find the old centre at West Drayton is letting the side down.

NATS has recently been partially privatised and already has serious financial problems brought on by the 11 September attacks and the crisis in the airline industry.

These faults manage to antagonise not only passengers but NATS' own shareholders - seven British airlines have a stake in the company.

One of them, Easyjet, was furious when the system went down two weeks ago because it badly affected short Easter breaks that many customers were planning.