Holidaymakers planning to jet out of Britain this summer face chaos caused by an air traffic control system blighted by mystery glitches.

There is nothing to stop the kind of fault that paralysed the new £600 million computer system, severely delaying flights last week, from happening again without warning. Controllers fear the fault could ground every single commercial and military plane waiting to take off in Britain or fly to or over the country.

This would throw air traffic across Europe into mayhem as flights were held at already crowded airports on the Continent, and those approaching the UK or flying overhead from the US were diverted.

Controllers said that when the system crashes engineers have less than seven minutes to work out whether the software has malfunctioned or there is a problem with the computer link between the new air traffic nerve centre at Swanwick in Hampshire and the old radar HQ at West Drayton, near Heathrow.

Even if the engineer makes the right diagnosis and the system only runs on manual for a matter of minutes, it causes hours of disruption. But if he makes the wrong choice or takes too long, the system shuts down and nothing can take off or land.

One West Drayton controller told The Observer this weekend: 'It is critical that when a problem crops up you choose the right recovery mode, otherwise everything would be grounded. Everyone is waiting to see what will happen over the next few months. There is no reason why this will not happen again.'

Engineers have not yet established precisely what caused the system to crash last Wednesday, nor how to prevent the same thing happening again, forcing controllers to switch to manual and record flight data by hand on paper strips.

Despite the new Swanwick radar centre opening in January - six years late and 50 per cent over budget - it still relies on a flight data print-out computer at West Drayton designed in the Seventies. This is prone to crashing when a message is mistyped or inputted in the wrong format.

Another controller said last week: 'While Swanwick continues to rely on West Drayton and the link to it, this will happen again and again. Lose the flight data system or the link and Swanwick goes manual - getting it back to electronic operation is not really possible until the traffic levels are reduced to a minimum.'

Although the system only crashed for 15 minutes at dawn last week, it caused delays for three hours.