Passengers are facing widespread flight disruption after a computer failure at the UK's air traffic control centre.
National Air Traffic Services (Nats) said a technical fault in the flight data system at its Swanwick centre had caused the problem.
It had now been restored to "full operational capacity", the air traffic control company said.
The government said the disruption was "unacceptable" and demanded a "full explanation" of what had gone wrong.
The glitch caused problems at airports around the country - including delays at Heathrow and Gatwick, where departing flights were grounded for a time. Other UK airports reported knock-on effects.
It comes a year after a telephone failure at the Hampshire control room caused huge disruption - one of a number of technical hitches to hit the part-privatised Nats since the centre opened in 2002.
The BBC's Lucy Manning said 84 flights were cancelled at Heathrow on Friday, and that delays are under an hour. The airport will stay open late but some disruption is expected tomorrow, she added.
Reported problems around the country included:
- Heathrow: Eighty-four out of about 1,300 flights cancelled but services gradually returning to normal
- Gatwick: Flights now resumed with delays of up to an hour-and-a-half after 19 earlier cancellations
- Stansted: Departures and arrivals experiencing some delays
- London City: Cancellations and delays
- Newcastle: All of Friday's departures have left after some delays
- Luton: Passengers told to expect "residual delays"
- Bristol: Delays reported but no cancellations
- Edinburgh: Experiencing some delays
- Glasgow: Some delays to arrivals and departures
- Southampton: All flights have now departed and landed after some delays
- Leeds Bradford: Flights now operational after earlier suspension
- Birmingham: Some departures are being re-routed to avoid flying through London airspace, causing some delays
- East Midlands: Departures and arrivals were delayed
- Manchester: Some delays of around an hour but no problems expected on Saturday
Nats said on Friday evening a "thorough investigation" was being carried out to "identify the root cause" of the disruption.
"Although operational restrictions applied during the failure have been lifted, it will take time for flight operations across the UK to fully recover so passengers should contact their airline for the status of their flight," it said.
"We apologise for the impact that this issue has had, and the delays and inconvenience caused."
Managing director Martin Rolfe earlier ruled both a computer hack and a power outage as possible causes.
Independent travel journalist Simon Calder said: "Tonight we have tens of thousands of people not where they want to be, extremely upset.
"And you've got aviation bosses absolutely furious that they have no liability for this terrible event but they have an immense responsibility to look after the passengers and that's going to cost them millions of pounds."
British Airways said if its customers did not want to travel from Heathrow, Gatwick or London City on Friday evening they could rebook or get a full refund.
EasyJet said it had cancelled 10 flights to and from Gatwick and would cancel a further two on Saturday.
Flight-tracking maps show Friday's disruption
Vicky Lane, a passenger on a grounded London to Dublin plane at Gatwick earlier, said: "We've been stuck on a Ryanair flight... for over an hour.
"The doors are open and we're really cold. I'm not sure when we will be leaving."
Another passenger, who was on a flight to Paris, said his plane had "circled around the Lake District for half an hour before turning back to Edinburgh".
Ed Bott told the BBC he was: "Currently sitting on the tarmac. None the wiser. Waiting for news as to what's happening."
Swanwick air traffic control centre
Swanwick controls the 200,000 square miles of airspace above England and Wales, cost £623m to build, and employs about 1,300 controllers.
But the facility, which handles 5,000 flights every 24 hours, has had a troubled history.
It opened in 2002, six years after its planned commissioning date - a delay which Nats said was due to problems with the software used to power its systems.
Almost a year after it opened, a senior air traffic controller raised concerns with the BBC about health and safety standards and complications with radio communications - which he said cut out erratically.
Technical problems and computer faults hit flights in 2008 and again last summer. And, in December 2013, problems with the internal telephone system then caused further delays.
Aviation journalist David Learmount said the IT problem would cause "major disruption" but would be resolved by Saturday.
"This impacts not just people within the UK, it impacts flights heading here from anywhere - anything heading this way will be told some of them can't be accepted, and they will have to go back to where they flew from or consider diverting to other countries," he told the BBC.
Flights going to London from Paris Charles de Gaulle airport have been delayed, and flights from Tunisia are landing at Charles de Gaulle instead of London.
Four outbound United Airlines planes have been held at the gate, but inbound flights have not been affected.
The RAF - which has its own air traffic control systems - said the UK military was unaffected.
By Ben Ando, BBC News
As soon as air traffic control has a problem, nothing is allowed to take off that might add to the problem.
This is unlikely to be a power problem as there are duplicate and back-up systems - including even diesel generators at a push.
It is more likely to be software, which caused the last major problem when the incoming morning crew could not switch over from night-time control system.
With aircraft out of position, or flight crews out of hours, there are likely to be knock-on problems for many flights.