An “unusual piece of data” caused widespread flight disruption, an air traffic control (ATC) boss said, as airlines are under growing pressure over their treatment of passengers.
Many UK holidaymakers are stranded overseas after around 2,000 flights were cancelled because of the issue.
There is speculation the ATC failure was caused by a French airline submitting a flight plan to National Air Traffic Services (Nats) in the wrong format.
Downing Street did not rule out that possibility, while Nats declined to comment on whether that was what happened.
Flights to and from UK airports were restricted for several hours on Monday afternoon as the fault prevented flight plans from being processed automatically, meaning manual checks were required.
Nats chief executive, Martin Rolfe, told BBC Radio 4’s Today programme: “It wasn’t an entire system failure. It was a piece of the system, an important piece of the system.
“But in those circumstances, if we receive an unusual piece of data that we don’t recognise, it is critically important that that information – which could be erroneous – is not passed to air traffic controllers.”
Mr Rolfe said Nats has “safety-critical systems” and “throwing data away needs to be very carefully considered”.
Willie Walsh, director-general of global airline body the International Air Transport Association (Iata) and former British Airways boss, described what happened as “staggering”.
He said: “This system should be designed to reject data that’s incorrect, not to collapse the system.”
An update from our CEO, Martin Rolfe on yesterday's air traffic control outage that we know continues to impact passengers around the world. pic.twitter.com/GtLeNE6rKf
— NATS (@NATS) August 29, 2023
Aviation analytics company Cirium said 64 flights due to serve UK airports on Wednesday were cancelled as of 9am, as the issue continued to have a knock-on effect with aircraft and crews in the wrong position.
There were 1,585 flights cancelled on Monday, while 345 were axed on Tuesday.
Many affected travellers are being told to wait several days for flights home.
Some have been forced to sleep on floors or makeshift beds at airports, or take long routes by land after their flights were cancelled.
Airlines were criticised for failing to book hotel rooms for many people who were delayed overnight.
Rob Bishton, joint interim chief executive of the Civil Aviation Authority, said airlines have “a responsibility to look after” passengers waiting to come home, which includes providing them with meals, refreshments and hotel accommodation.
He told affected travellers: “If airlines cannot do this, you can organise your own meals and accommodation, then claim costs back.
“We are engaging with airlines and know that more flights are being provided, but in circumstances where this has not been possible due to the volume of passengers, consumers can book their own alternative air travel and claim the cost back from their airline.”
Transport Secretary Mark Harper said on Tuesday night that “airlines are clear about their responsibilities to their customers”.
Rory Boland, editor of consumer magazine Which? Travel, said airlines are “failing to properly communicate with their passengers or fulfil their legal obligations”.
And Rory Dollard, 40, a cricket journalist at PA Media, said he and his family will be stuck in France until Sunday and must fly back from an airport hours away from where they are staying.
Their Ryanair flight home from Bergerac Dordogne Perigord Airport on Monday was cancelled.
Mr Dollard said: “Six days, it’s remarkable really. I’ve been to the airport again today and the flights had restarted for Ryanair but they were all full already so it wasn’t a case that we could book on to the next flight.”
Matthew Creed, a 26-year-old drama student from Harthill, North Lanarkshire, became stuck at Schiphol Amsterdam Airport after his flight with KLM Royal Dutch Airlines to Edinburgh was cancelled.
Mr Creed said it “wasn’t ideal” sleeping on a folding bed at the airport, and he was unable to speak to airline staff as “all the desks were closed”.
EasyJet is operating five repatriation flights to Gatwick, bringing people home from destinations in Spain, Portugal, Greece and Tunisia.
It is also using larger aircraft on other busy routes to boost capacity.
Mr Rolfe said Nats is working closely with the Civil Aviation Authority to provide a preliminary report into what happened to Transport Secretary Mark Harper.
The conclusions of the inquiry will be made public, he added.
Mr Walsh estimated that the chaos will cost airlines around £100 million.
He said: “It’s very unfair because the air traffic control system which was at the heart of this failure doesn’t pay a single penny.”