The air traffic control failure which led to major flight disruptions this week was caused by flight data received by National Air Traffic Services (Nats), officials have said.
Both primary and back-up systems responded to the data by suspending automatic processing, Nats chief executive Martin Rolfe said in a statement on Teesday
There are no indications the failure was caused by a cyber-attack, he added.
The issue started on Monday, when more than a quarter of flights at UK airports were cancelled.
Mr Rolfe said on Tuesday he wanted to “reassure” passengers that all Nats systems had been running normally since Monday afternoon, to support airline and airport operations.
“Very occasionally technical issues occur that are complex and take longer to resolve,” he said.
“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, 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.”
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.
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.”