The air traffic control failure which led to a spate of flight cancellations was caused by flight data received by National Air Traffic Services (Nats), with both primary and back-up systems responding by suspending automatic processing, chief executive Martin Rolfe said.
There are no indications that the failure was caused by a cyber-attack, he added in a statement released on Tuesday.
Mr Rolfe also wanted to “reassure” people that all Nats systems have been running normally since Monday afternoon to support airline and airport operations.
He said: “Very occasionally technical issues occur that are complex and take longer to resolve.
“In the event of such an issue our systems are designed to isolate the problem and prioritise continued safe air traffic control.
“This is what happened yesterday.
“At no point was UK airspace closed but the number of flights was significantly reduced.
“Initial investigations into the problem show it relates to some of the flight data we received.
“Our systems, both primary and the back-ups, responded by suspending automatic processing to ensure that no incorrect safety-related information could be presented to an air traffic controller or impact the rest of the air traffic system.
“There are no indications that this was a cyber-attack.
“We have well established procedures, overseen by the CAA (Civil Aviation Authority), to investigate incidents.
“We are already working closely with them to provide a preliminary report to the Secretary of State for Transport on Monday.
“The conclusions of this report will be made public.”
The issue started on Monday, when more than a quarter of flights at UK airports were cancelled.
Nats suffered what it described as a “technical issue”, preventing it from automatically processing flight plans.
This resulted in flights to and from UK airports being restricted while the plans were checked manually.
Nats said at 3.15pm on Monday the problem was resolved, but disruption continued into Tuesday as many aircraft and crews were out of position.
Analysis of flight data websites by the PA news agency shows at least 281 flights – including departures and arrivals – were cancelled on Tuesday at the UK’s six busiest airports.
This consisted of 75 at Gatwick, 74 at Heathrow, 63 at Manchester, 28 at Stansted, 23 at Luton and 18 at Edinburgh.
EasyJet announced it will run five repatriation flights to Gatwick following the air traffic control fault as well as operating larger aircraft on key routes.
It 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.”
Aviation analytics company Cirium said 790 departures and 785 arrivals were cancelled across all UK airports on Monday.
That was equivalent to around 27% of planned flights and means around a quarter of a million people were affected.
British athletes were stranded in Budapest after the World Championships.
A group of around 40 athletes and staff from UK Athletics returned to their hotel in the Hungarian capital on Monday night because of the flight chaos.
Some of the affected athletes chose to travel directly to Zurich for Thursday’s Diamond League event.
Holidaymakers stuck in the UK and abroad described their frustration, as some had no idea when or how they would get to their destination.
Katrina Harrison and her family – including one-year-old twin grandchildren – spent the night at Leeds/Bradford Airport after their flight to Antalya, Turkey, was cancelled on Monday afternoon.
Ms Harrison, from Stockton-on-Tees, told the PA news agency: “There were no hotels to stay in, we couldn’t get the car out of the car park.
“We haven’t slept, we tried to sleep on the floor but couldn’t. Luckily the children could sleep in the pram.
“The holiday was supposed to be a family celebration of a few things. We’ve spent £12,000 on it and we’ve been treated like muck.”
Ryan and Kirsty Fawcett, from Selby in North Yorkshire, were at East Midlands Airport with their two-year-old twin sons for their first holiday as a family – and the couple’s first since 2019 – to Antalya in Turkey.
Their flight on Monday afternoon was cancelled, and after staying in a hotel overnight, they were booked on another departure on Tuesday morning that was also axed.
Ms Fawcett said: “What has annoyed us more is we have been told ‘just sit and wait around’, with the extra expense of hotels and things.
“What if we didn’t have money spare?”