Most flights from Heathrow airport were grounded this morning after a computer failure at the National Air Traffic Control centre at West Drayton.
Air traffic controllers reverted to manual handling of aircraft in the air as engineers raced to restore IT systems.
Many airports are advising people to check in as normal, but delays are likely to continue throughout the day.
A spokesman for National Air Traffic Services said the fault was believed to be in data flow through the systems that feed Nats' £623m Swanwick centre in Hampshire.
This morning's failure is a further embarrassment for Nats. In January, Computer Weekly revealed that Nats was to upgrade software at Swanwick after a near-miss of two large passenger jets over Wales, during which a woman was injured.
The incident occurred after an air traffic controller confused the positions of a Virgin Atlantic 747-400 jumbo and a Delta Airlines 767 on his screen.
The controller thought his instructions to the pilots of the aircraft would direct their aircraft away from one another, but the opposite happened.
Last June, Computer Weekly revealed that a top-level safety report on the Swanwick centre revealed that technological problems contributed to a record number of overloads for air traffic controllers in 2002.
The report said that overloads more than doubled after the Swanwick En Route Centre went live in January 2002.
"A number of problems and shortcomings associated with the technology used by the controllers were cited as creating an environment where overloads were more likely," said the Nats safety review committee report, which was commissioned by its board of directors.
By law, controllers must report an overload if their workload is excessive to the point where the safety of aircraft was, or could have been, compromised. The safety committee said there were 64 overloads in 2002, compared with 28 when en route aircraft were handled by the centre's predecessor at West Drayton, near Heathrow.