Major flight disruption which has left many holidaymakers stranded overseas was caused by a problem with inbound data, air traffic control (ATC) bosses have said.
National Air Traffic Services (Nats) chief executive Martin Rolfe said initial investigations show the failure “relates to some of the flight data we received”.
Primary and back-up systems used by the company responded by “suspending automatic processing”, he added.
The impact continued on Wednesday with at least a further 42 flights to or from Heathrow cancelled.
Many affected travellers are being told to wait several days for flights home.
Some have been forced to sleep on airport floors 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.
Rory Boland, editor of consumer magazine Which? Travel, said: “We’re seeing worrying reports of passengers being left stranded without support, and airlines failing to properly communicate with their passengers or fulfil their legal obligations such as offering timely rerouting or providing overnight accommodation.
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
“In particular, travellers should be aware that their airline has a responsibility to reroute them as soon as possible, even if that means buying them a ticket with a rival carrier – a rule that some airlines appear to be ignoring.
“Passengers should also be given food and refreshments and overnight accommodation if required.”
EasyJet is operating five repatriation flights to Gatwick, with the first two setting off on Wednesday.
The airline said: “During this traditionally very busy week for travel, options for returning to the UK are more limited on some routes and so easyJet will be operating five repatriation flights to London Gatwick over the coming days from Palma and Faro on August 30, and Tenerife and Enfidha on August 31, and from Rhodes on September 1.
“We are also operating larger aircraft on key routes including Faro, Ibiza, Dalaman and Tenerife to provide some additional 700 seats this week.”
There is speculation the ATC failure was caused by a French airline submitting a flight plan to Nats in the wrong format.
Downing Street did not rule out that possibility, while Nats declined to comment on whether that was what happened.
The problem resulted in flights to and from UK airports being restricted for several hours on Monday afternoon while flight plans were checked manually.
This caused the cancellation of around 1,500 flights on Monday, with a further 300 axed on Tuesday due to aircraft and crews being out of position.
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.
Willie Walsh, director-general of global airline body the International Air Transport Association (Iata), described what happened as “staggering”.
He told BBC Radio 4’s Today programme: “This system should be designed to reject data that’s incorrect, not to collapse the system.”
Mr Rolfe said Nats has “safety-critical systems” and “throwing data away needs to be very carefully considered”.
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.”