Tens of thousands of airline passengers in Britain faced hours of delay after a computer failure at the country's main air traffic control centre grounded many flights.
Heathrow, Gatwick, Manchester and Stansted airports were all affected by the computer glitch, with passengers warned to check with airlines before leaving home.
"There is serious disruption at Heathrow as a result of the air traffic control situation. There is an average of two-hour delays on departures and restrictions on arrivals. We are advising passengers to check with their airlines before leaving home," said Mark Pearson, spokesman for Heathrow Airport which handles 1,250 flights a day.
The national system went down at around 6 am (1030 IST) and controllers had to switch to manual operations. National Air Traffic Services (NATS) chief Richard Everitt said the failure followed overnight testing of an upgrade to its Flight Data Processing System in West Drayton.
"This is a significant upgrade that we will be doing later in the year, we have to test that very thoroughly because safety is paramount. We will now investigate why there was this problem - clearly it was not an anticipated problem - a lot of work will be done today to understand why we had problems with this testing," he said.
Everitt said NATS was planning to spend over one billion pounds on upgrading its system over the next eight years.
He apologised to passengers and said NATS was working with airlines to minimise disruption. The privatized NATS, the firm that operates air traffic control in the UK, said passenger safety was never put at risk despite the computer crash.
NATS spokesman Adian Yalland said they believed they had located the problem at an operations centre in West Drayton, near Heathrow Airport and the system was running again. Gatwick was operating around 10 outbound flights per hour, but would normally handle 30 to 40. Arrivals were not now affected.
Stansted in Essex was operating at round 60 per cent capacity, according to airport operator British Airport Authorities.
In Scotland, flights to London and Bristol were grounded, but BAA said services to other parts of the UK were beginning to restart.
