Thousands of travellers were delayed this morning after a key air traffic control system failed for the second time in two weeks.

The Flight Data Processing System (FDPS) at the West Drayton centre went down at 6am. Although it was up and running by 6.40am, air traffic control was expected to run at 70 per cent of capacity until 10am.

The system prints the paper strips that controllers use to plot the movements of planes. When the system fails the strips have to be written out manually. The same system failed two weeks ago for 20 minutes, causing similar delays.

Although the hardware for the FDPS was upgraded from an IBM 4381 to an IBM 0/S390 in Autumn last year, the application still contains some code created in the 1970s.

National Air Traffic Services, which runs air traffic control, said that there are software upgrades to the system around once a month.

The same system crashed four times in 2000 causing massive delays, and is not scheduled for replacement until 2007.

Airspace over London and the south east up to 20,000 feet is still controlled from the site at West Drayton.

The control of high level airspace is now done from the air traffic control centre at Swanwick, which opened in January five years late and £180m over budget.