The air traffic control issue that led to widespread disruption to UK air travel was not caused by a cyber-attack, the UK government has said.
More than a quarter of all UK flights were axed on Monday after a fault with National Air Traffic Services (Nats).
Delays and cancellations are continuing on Tuesday, as passengers scramble to get on flights.
The PM’s official spokesperson said the incident will now be investigated by the Civil Aviation Authority (CAA).
“The information we have is that there was not a cyber-attack,” they added.
“The exact cause of the technical incident will be subject to the investigation by the Civil Aviation Authority and then submitted to government.”
Earlier, Transport Secretary Mark Harper said it will take “some days to get completely everybody to where they should be”, after thousands of people were left stranded by the technical glitch.
“The last time there was something this significant was about a decade ago,” Mr Harper told BBC Radio 4’s Today programme on Tuesday morning.
He admitted the timing “was not at all helpful for people” – but that “those people in government who look at these things have looked at it and are clear that it wasn’t a cyber attack”.
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.
Prime Minister Rishi Sunak said Mr Harper was in “constant dialogue” with all the industry participants and will be talking to airlines on Tuesday.
Heathrow Airport – the world’s busiest two-runway airport – said on Tuesday that its schedule remained “significantly disrupted”, while EasyJet said some flights are still “unable to operate”.
Holidaymakers described a nightmare Bank Holiday Monday, with many waiting hours for news of when they might get moving.
National Air Traffic Services (Nats) confirmed the fault just after midday on Monday, before it announced at 15:15 BST that it had identified and remedied the issue.
But it said it would “take some time for flights to return to normal” – as it launched an investigation into what went wrong.
Aviation expert John Strickland explained the knock-on impact on flight disruption will continue to “ripple on over the next few days”.
“Fortunately it is very uncommon,” Mr Strickland told BBC Radio 4’s Today programme. “The scale of this one was so significant – instead of minutes it lasted into hours.
“Hours really do have an enormous domino effect.”
According to aviation data firm Cirium, 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.
Passengers have recounted how they faced huge disruption as air traffic control had to input routes manually, rather than automatically, because of the fault.
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.”
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 have been 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 customers due to travel on a short-haul service on Tuesday may be able to move their flights free of charge to a later date.
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 that 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 and announced an investigation into what happened.