An air traffic control boss has said "unreliable" flight data was the cause of widespread disruption that affected thousands of passengers stuck at airports.
Martin Rolfe, the chief executive of National Air Traffic Services (NATS), said an initial investigation had found the air traffic control failure was caused by flight data which its system "didn't understand" and "couldn't interpret".
Mr Rolfe added such "unreliable" data was kept away from air traffic controllers so they didn't "act in an unsafe way" - so NATS reverted to manual systems.
He said this allowed them to "continue operating but at a much lower capacity, handling less flights".
The failure is estimated to have cost airlines £100m, according to the head of the International Air Transport Association (IATA), Willie Walsh.
More than 1,000 flights departing UK airports have been cancelled over the past three days including at least 30 by 9am today, data from aviation analytics company Cirium revealed.
Meanwhile, 983 arrivals into UK airports have been cancelled, including 34 on Wednesday by 9am.
Mr Walsh said the air traffic control system "should be designed to reject" incorrect data and not cause it to "collapse", while speaking to the BBC.
But Mr Rolfe said rejecting flight data was "nothing like throwing away spam" for air traffic controllers.
He added: "If you throw away a critical piece of data you may end up - in the next 30 seconds, a minute or an hour - with something that then is not right on the screens in front of the controller."
Mr Rolfe previously told Sky News: "You will understand we have very complex systems, handling something in the region of two million flights a year and the safety of those passengers is incredibly important to us."
Reports have suggested the chaos may have been caused after a French airline misfiled its flight plan.
Without confirming the reports, Mr Rolfe said: "It could be a single flight plan... if it is a flight plan that has caused this, we know it is something in the flight data and we will get to the bottom of it and understand why.
"We are conducting an investigation, we will conduct it incredibly thoroughly."
NATS said earlier there was "no indication" it was targeted in a cyber attack.
Regardless of the cause, Mr Walsh said the compensation situation was "unfair" for airlines, as the air traffic control system "at the heart of this failure doesn't pay a single penny".
Chaos continued into second day
Thousands of passengers have been affected - and many are still waiting for their flights.
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.
Cirium said a total of 799 departures and 786 arrivals were cancelled across all UK airports on Monday - equivalent to around 27% of planned flights, affecting around a quarter of a million people.
A further 345 departing or arriving flights were cancelled on Tuesday - around 6% of all those into and out of the UK.
EasyJet announced it would run five repatriation flights to Gatwick following the air traffic control fault as well as operating larger aircraft on key routes.
'I have never felt so helpless'
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.
Vicki Ostrowski emailed Sky News to say she was stranded in Oslo with a "disabled, wheelchair-bound passenger with a neurological disease, an 83-year-old frail relative, plus three other family members".
Kayleigh, another reader, got in touch to say she was stuck at Las Palmas airport in Gran Canaria.