Airports across the UK grounded flights on Wednesday afternoon when a major fault hit air-traffic control.
Radar systems went down as authorities investigated a glitch in the systems serving what is known as the London Control Area.
Heathrow and Gatwick airports said take-off and landings were suspended. There was a knock-on effect across the country, with Edinburgh, Manchester, Birmingham, Liverpool, Stansted and East Midlands reporting flights grounded.
Within minutes, passengers were reporting widespread delays and cancellations affecting flights.
Nats, the company that operates most UK air-traffic services, said the issue was resolved about an hour after it was first reported and is in the “process of resuming normal operations”.
Delays to domestic and international flight services are still expected. While the issue was ongoing, British Airways said the problem was “affecting the vast majority of our flights”.
In an earlier statement, Gatwick Airport said: “A technical issue impacting Nats is affecting all outbound flights across the UK. There are currently no departures from London Gatwick while the situation is being resolved."
Three years ago, a technical glitch occurred for the National Air Traffic Services after a "fail-safe" mode activation could not reboot. The disruption lasted for days.
Earlier on Wednesday, emergency vehicles attended a United Airlines passenger jet at Heathrow when it returned after issuing a distress call shortly take-off. The airline said it was evaluating a technical issue after smoke was reported in the cabin on Flight UA949.
The San Francisco-bound plane was only minutes into its flight over the English city of Milton Keynes when it turned back. The emergency landing at Heathrow went smoothly. The reports of smoke in the cabin were confirmed but no details of passenger welfare were immediately available.
The model is believed to be a Boeing 777-200ER. In June, a similar plane was forced to land in Iceland during a flight to Rome. It had flying at 21,000 feet when it made a distress call to the mid-Atlantic airport.
