Watch: The day UK air traffic control went down... in 71 seconds
An investigation has been launched over UK flight disruption that has left thousands of passengers stranded.
The National Air Traffic Services apologised after a technical issue on Monday led to cancellations and delays.
The disruption has seen passengers sleeping overnight on airport floors, or forking out thousands of pounds for alternative routes.
Air traffic control bosses have "crucial questions to answer", the head of one industry body said.
Willie Walsh, head of the International Air Transport Association, called the failure "unacceptable".
The incident will be investigated by the Civil Aviation Authority (CAA).
Nats first confirmed the issue just before midday on Monday, and about three hours later said it had identified and remedied the issue.
It said it would "take some time for flights to return to normal" and it was investigating what went wrong.
Passengers have been warned to expect knock-on disruption in the coming days.
Transport Secretary Mark Harper echoed Nats' apology, and said his priority "continues to be making sure passengers get where they need to be as quickly as possible".
Mr Walsh, of the IATA, said he feels for passengers who continue to suffer "huge inconvenience" and airline staff put under "considerable additional stress".
He said: "Nats has crucial questions to answer about their responsibility for this fiasco.
"The failure of this essential service is unacceptable and brings into question the oversight of the CAA who are required to review the Nats resilience plan under the terms of its licence."
He added that airlines would "bear significant sums in care and assistance charges, on top of the costs of disruption to crew and aircraft schedules. But it will cost Nats nothing."
Nats has said the problem was a "technical issue" with its flight planning system which meant the plans had to be processed manually, which could not be done at the same volume, so it had to reduce the number of planes flying.
But it has not said what caused the problem.
The PM's official spokesperson said the exact cause would be investigated by the Civil Aviation Authority and then submitted to government.
"The information we have is that there was not a cyber-attack," they added.
Asked about reports that a French data entry error was responsible for the outage, the spokesperson said they would not comment on "speculation", adding: "I'm not going to pre-empt the work that needs to be done."
They added that they were "not aware of any specific conversations" between French counterparts and officials.
Passengers have recounted how they faced huge disruption because of the issue.
Katrina Harrison and her family - including one-year-old twin grandchildren - spent the night at Leeds Bradford Airport after their flight to Antalya was cancelled on Monday afternoon.
Ms Harrison, from Stockton-on-Tees, told the PA news agency they were given a bottle of water, a KitKat and a packet of crisps and all the shops sold out of food and drink on Monday night.
"We weren't given a blanket, we've been absolutely freezing," she said. "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."
Aviation data firm Cirium said that as of 9:00 BST on Tuesday, 147, or 5%, of UK departing flights had been cancelled as well as 134, or 5%, of arrivals.
Heathrow Airport - the world's busiest two-runway airport - said on Tuesday that its schedule remained "significantly disrupted".
EasyJet had been disrupted on Tuesday but by the evening said it was now operating normally.
Aviation analyst Sally Gethin said the disruption in the aftermath of the technical fault will last for days, saying there could be a "knock-on effect into later this week".
Cirium said 790 departing flights were cancelled on Monday, which it said was equivalent to about 27% of all departures, and 785, or about 27%, of incoming flights.
Heathrow had the highest number of cancellations, Cirium said, followed by Gatwick and Manchester.
Serena Hamilton at Belfast International Airport said she was likely to miss a heart transplant check-up after her flight to Newcastle-upon-Tyne was cancelled.
"I had a transplant 15 months ago and these appointments are very important," she told BBC News.
Cricket journalist Rory Dollard and his family are stuck in Bergerac, France and were told it could take up to six days to get home after his Ryanair flight was cancelled.
Thousands of passengers hit
Airports and airlines were forced to apologise to travellers for the delays and cancellations, and in some cases have offered passengers full refunds.
Michael O'Leary, boss of Ryanair, said they had to cancel about 250 flights on Monday, affecting about 40,000 passengers. On Tuesday a further 70 flights were cancelled, he said, adding that they were hoping to run a "normal operation" on Wednesday with "minimal delays".
"It is sadly outside of our control," Mr O'Leary said. "We have been in contact with UK Nats, we still haven't had an explanation from them - what exactly caused this failure yesterday? And where were their back-up systems? It's not acceptable."
BA said there were "significant and unavoidable delays and cancellations" and apologised for the inconvenience caused. It has advised customers who were travelling on short-haul services to check their flight was still running before heading to the airport.
The airline added that customers due to travel on a short-haul service on Tuesday or Wednesday may be able to move their flights to a later date free of charge.
In a statement late on Monday night, Tui reassured its customers that on top of a refund they would be entitled to a "future holiday voucher of £100 per person".
The CAA says an airline has a duty of care to provide food, drink and accommodation if delays stretch overnight.
If a flight is cancelled, passengers should be offered a choice of a refund or alternative travel arrangements at the earlier opportunity.
Nats said it was a "flight planning issue" which had affected the system's ability to automatically process flight plans.
This meant that flight plans could not be processed at the same volume, "hence the requirement for traffic flow restrictions" for safety.
Operations director Juliet Kennedy apologised for the disruption.
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