Flights have resumed in the United Kingdom following major disruption caused by an air traffic control issue.

The National Air Traffic Service said the fault occurred at 4:05 p.m. at its Swanwick control centre and was resolved within 20 minutes., according to Sky News.

The agency has apologised for the incident.

“This was a radar-related issue which was resolved by quickly switching to the back up system during which time we reduced traffic to ensure safety,” said a spokesperson.

“There is no evidence that this was cyber related.”

Earlier, Gatwick Airport reported that the issue was “affecting all outbound flights across the UK,” while Birmingham Airport said departures from many UK airports had been suspended.

Shortly after 5 p.m., NATS confirmed that departures had resumed nationwide and efforts were underway to clear the backlog.

As of 7:30 p.m., a total of 67 departures and 55 arrivals had been cancelled nationwide, according to aviation analytics firm Cirium.

A Heathrow spokesperson advised passengers to check with their airlines before heading to the airport.

According to Flightradar24.com, by 5:30 p.m.—an hour after NATS restored its systems—departing flights at Heathrow were experiencing average delays of 48 minutes, with 89% of flights affected.

British Airways temporarily limited inbound and outbound flights at Heathrow to 32 per hour but said operations would return to the usual 45 per hour from 7:15 p.m.

The outage drew strong criticism from some airlines.

Ryanair called for the resignation of NATS Chief Executive Martin Rolfe, claiming “no lessons have been learnt” since the August 2023 system failure that disrupted flights for over 700,000 passengers.

Chief operating officer Neal McMahon called Wednesday’s fault “outrageous”.

“Yet another ATC system failure has resulted in the closure of UK airspace, meaning thousands of passengers’ travel plans have been disrupted,” he said.

EasyJet Chief Operating Officer David Morgan also expressed concern over the disruption, adopting a serious tone in his response.

“It’s extremely disappointing to see an ATC failure once again causing disruption to our customers at this busy and important time of year for travel,” he said.