A computer failure caused widespread disruptions in airline service around London on Friday, grounding flights and stranding passengers at one of the busiest times of the week.
Britain's air traffic control service confirmed a "technical failure" at its Swanwick center in southern England, but said the issue had been resolved.
"The system is back up and running now," Martin Rolfe, managing director for operations at NATS Holdings, told the BBC.
He said the cause of the fault was being investigated, but said it was not due to a power outage or a cyber attack.
"Obviously technical systems do fail at times," he said.
The BBC reported that the problem was caused by a software glitch affecting the coordination of flights coming into and taking off from London.
The skies were unusually empty over London on Friday afternoon. British airspace was not closed but capacity was restricted during the incident, air traffic control officials said.
All flights departing from
Although departures resumed in less than an hour at both airports, officials warned that it would take time for operations to fully recover and delays could continue into Saturday. They urged passengers to contact their airline for information about their flight status.
At least 66 flights were canceled at Heathrow and the figure was expected to rise, airport officials said. At Gatwick, about five flights were canceled and dozens were delayed.
The flight disruptions left some passengers stuck on the tarmac for hours, with the ripple effects felt across Britain and beyond.
"Get me OFF here. Stuck on Brussels plane Heathrow-bound, haven't taken off and we have a wait of over 3 hours before we can," tweeted @DeborahLincoln.
Special correspondent Boyle reported from London and staff writer Zavis from Los Angeles.