A failure of the National Air Traffic Service's (Nats) West Drayton flight data processing system has grounded flights across the UK. The crash occurred following overnight testing for a future computer upgrade.

A spokeswoman for Nats said that the system was taken down overnight for testing, but crashed when technicians tried to bring it back online this morning.

"The problem occurred at 6.03am, at which point we tried to reboot the system. We got it up again at 6.42am and it was fully operational by 7.03am, at which point we tested the system's robustness. Normal operations resumed at 8.05am," she said.

"During this period all flights were able to land as normal, but we restricted take-offs as a safety precaution. Our air traffic controllers were still able to see all planes on their screens and communicate with them."

The spokeswoman added that the upgrade was due to go ahead in August as part of Nats' £1bn modernisation programme to upgrade its entire air traffic control operation.

Speaking on Radio 4's Today programme, Transport Secretary Alistair Darling warned passengers of "a lot of disruption".

The system went down at 6am and technicians took about 45 minutes to get it back online. Although it is now operational again, major flight delays are expected for the rest of the day as airports work to clear the backlog.