A computer failure left passengers stranded at airports across the UK this weekend as air traffic control staff resorted to identifying aeroplanes and their destinations through hand-written notes.

The National Air Traffic Services (Nats) said its flight data processing system failed to process and print 'flight strips' - which tell air traffic controllers information about each aeroplane and its destination - between 9.30am and 1.15pm on Saturday.

The computer failure occurred at the West Drayton traffic control centre near Heathrow Airport.

Nats said it regretted the inconvenience caused by the problem but stressed that safety had not been compromised at any time. A spokeswoman said air traffic controllers (ATCs) had full radar displays at all times on Saturday and were in radio communication with all aircraft.

The delays were because the combination of human, pen and paper used to cover for the computer failure could not process information as quickly as computers, so less traffic was allowed in the air.

"Because part of the ATC system had to be controlled manually, we had to restrict the number of flights through UK air space," said the spokeswoman.

It is the second time this has happened this month. An identical system failure occurred between 8.30am and 8.50am on 9 June.

A Nats spokeswoman said they were working flat out to identify the latest problem and prevent further delays, but as millions of data lines had to be checked she could not give an indication of when the reasons for the system failure might be identified.