It is a very creaky system that has been patched together over a number of years

Iain Findlay, Prospect union

The computerised system was fully operational again by around 35 minutes later, but air traffic control was forced to run at 70% of its usual capacity until about 1000 BST.

After initial delays of up to two hours, most UK airports were able to operate a normal service by mid-morning.

But some flights were still landing up to an hour late in the afternoon.

Software problems

Air traffic control

Swanwick controls most of the 200,000sq miles of airspace across England and Wales West Drayton has control over the area below 24,000ft around London and the South East A small control centre in Manchester covers the area below 21,000ft surrounding the city Scottish air traffic is controlled by two centres at Prestwick in Ayrshire

The system first broke down in the run-up to the busy Easter bank holiday, and left thousands of travellers stranded in airports around the world.

Richard Everitt, chief executive of National Air Traffic Services (Nats) told the BBC that particular incident had not been caused by a computer glitch.

It had been caused by the incorrect inputting of data, he said.

Neither problem was connected with computers at the new Swanwick centre, which was dogged by software problems before it opened six years late in January.

'Patched together'

Iain Findlay, a spokesman for the air traffic controllers' union, Prospect, described the computer system at West Drayton as "creaky" and said an investigation must establish why it had broken down again.

Safety "was not compromised"

He told BBC Radio 4's Today programme: "It is a very creaky system that has been patched together over a number of years. It does seem to be susceptible to crashing."

He stressed the breakdown did not compromise safety.

"Planes are still seen on the radar, they know where every plane is and they are being kept a safe distance apart."

These incidents are a further setback for the newly-privatised Nats, which has had to receive the �30m bail-out from the government after running into financial difficulties following the events of 11 September.

'Repeated breakdowns'

A group of seven airlines, including British Airways and Virgin Atlantic, has taken a 46% stake in Nats.

On Tuesday, one of these partner airlines - low-cost carrier easyJet, said it was considering writing off its investment.

Shadow transport secretary, Theresa May, said the incident showed the new organisation's 'flawed structure'.

She is calling on the Transport Secretary Stephen Byers to reassure passengers that "the repeated breakdowns are not a fundamental deterioration in the service provided under the new Nats".