Grounded planes were on Monday afternoon set to take to the air again, after air-traffic control in the UK was hit by a technical fault.
One airline had reported a “network-wide failure”, while the National Air Traffic Service said it had “applied traffic flow restrictions to maintain safety” while its engineers worked to fix the issue.
Nats explained the technical issue meant flight plans had to be entered manually, slowing down the flow of air traffic considerably.
It announced the problem had been resolved by midafternoon but Transport Secretary Mark Harper said on Monday evening that flights were still being affected.
More than 500 flights had been cancelled as of 2.30pm UK time, the BBC reported, citing data from aviation analytics firm Cirium.
“We have identified and remedied the technical issue affecting our flight-planning system this morning,” said Nats. “We are now working closely with airlines and airports to manage the flights affected as efficiently as possible.
“Our engineers will be carefully monitoring the system’s performance as we return to normal operations.”
Juliet Kennedy, the operations director at Nats, later tweeted an apology and said the failure would be thoroughly investigated.
“First of all, I’d like to apologise for the impact on people’s travel plans today," Ms Kennedy said.
“The issue we had earlier meant that our automatic system, which provides controllers with details of every aircraft and its route, wasn’t working.
"Instead, to manage safety, we had to limit the number of flights we could manage.
“Our teams worked hard to resolve the problem, and I’m pleased to say it was fixed earlier on this afternoon. However, it will take some time for flights to return to normal.
“And we will continue to work with the airlines and the airports to recover the situation. Our absolute priority is safety and we will be investigating very thoroughly what happened today.
“Again, I would like to apologise for the impact on the travelling public and to tell you that our teams will continue to work to get you on your way as soon as we can.”
Knock-on effects could last days
Travel expert Simon Calder said while there was no immediate danger because the air-traffic control system is designed to cope with such situations, there would be a knock-on effect throughout Europe that could last for hours or even days in what is one of the busiest times of the year.
“There is going to be absolute disruption and chaos spreading across Europe,” he told Sky News.
“If you are on the ground at Heathrow or Gatwick, which are the busiest two-runway and busiest single-runway airports in the world, there is very, very little slack in the system.
“Meanwhile, there are thousands of aircraft in the skies above Europe, hundreds of them heading to the UK. Some of them are getting down, if they are in the vicinity of the airfield, then they will be landing.
“But otherwise you are going to start seeing aircraft either being held on the ground in airports such as Amsterdam – or indeed being diverted if they’re on a longer flight, and that would typically be a continental airport.”
He said that even though the fault was fixed relatively quickly, “it will have caused enough disruption for the system to be in disarray for certainly the rest of the day and possibly for further days”.
Earlier, Irish air-traffic control provider AirNav Ireland said the issue, which struck during a public holiday in parts of Britain, was resulting in “significant delays for flights across Europe that are travelling to, from or through UK airspace”.
A representative for London Heathrow, the busiest airport in Western Europe, said it was working with Nats and other partners to minimise the impact on passengers, while Gatwick, south of London, said reported numerous delays and cancellations.
Emirates airline said on Monday: “All flights currently en route to the United Kingdom will be allowed to land, but flights departing the UK may experience delays.”
The airline added that it was continuing to “monitor the situation apologises to its customers for the inconvenience”.
Etihad Airways said it was “advising passengers travelling to and from London and Manchester that there may be flight delays due to the ongoing Flight Data Processing System failure affecting all airlines operating in the UK”.
“All flights are currently planned to operate as scheduled, however, we are working closely with airport authorities and will inform guests immediately if there is any change,” said the airline.
“We apologise for the inconvenience caused by this system failure which is affecting all airports across the UK.”
Scottish airline Loganair was the first to reveal there were difficulties, saying there had been “a network-wide failure of UK air-traffic control computer systems”, with flights grounded.
“Although we are hopeful of being able to operate most intra-Scotland flights on the basis of local co-ordination and with a minimum of disruption, north-south and international flights may be subject to delays,” it said late on Monday morning.
Broadcaster Gabby Logan said on X, formerly known as Twitter, that she was on a plane on a runway at Budapest International Airport, and had “just been told UK airspace is shut”, adding “we could be here for 12 hours”.
EasyJet passengers had been told the issue was affecting all flights in and out of Britain.
“We have been advised of an air-traffic control issue currently affecting all flights due to fly into or out of United Kingdom airspace,” the airline said before the problem had been resolved.
British Airways said it was “working closely with Nats to understand the impact of a technical issue that is affecting UK airspace and will keep our customers up to date with the latest information”.
Tui warned of “significant delays”, while Ryanair passengers said they had been told to wait at their gate until further notice.