London airspace has been thrown into chaos after a power outage temporarily closed a control centre on Friday afternoon.
National Air Traffic Services (Nats) confirmed the glitch at its headquarters but warned passengers to expect delays.
They said: "Following a technical failure at Swanwick, the system has been restored and we are in the process of returning to normal operations.
"We apologise for any delays and the inconvenience this may have caused.
"Further information will be released as it becomes available."
Heathrow said flights were now departing and arriving, but urged people to check their flight status before travelling to the airport.
London Luton Airport said: "Flights now resuming but please expect residual delays."
Bristol Airport said: "Flights have resumed after earlier UK Air Traffic Control problems, though there may be delays. If you're flying, please check in as normal."
Newcastle Airport said: "If you're travelling with us this evening please come to the airport as normal, however we do expect there will be some delays."
Stansted, was still accepting arrivals but had suspended departures. City was resuming normal service with delays. 50 planes had been "impacted".
Planes are departing Gatwick but are subject to delays.
A Heathrow spokesman said that 50 flights had had to be cancelled at the west London airport and that he expected that figure would rise as the evening went on.
He went on: "The earlier problem will cause delays and cancellations to flights for the rest of the day and is likely to have a knock-on effect on some services tomorrow because aircraft and crew will now be out of position.
"Passengers due to depart today and tomorrow should check the status of their flight with their airline before travelling to Heathrow. We are very sorry for the disruption to passengers' journeys. We have extra staff on duty to help passengers."
The glitch lasted from 3.27pm to 4.03pm. The fact that Heathrow typically handles between 80 and 90 flights an hour shows how even a short disruption to take-offs and landings can have a big effect.
Initially air traffic management company Eurocontrol announced that there would be no flights at UK airports until 7pm.
But shortly after 4pm Nats said: "UK airspace has not been closed, but airspace capacity has been restricted in order to manage the situation."
For a time there were no flights able to land or take off at Heathrow or Stansted following the computer glitch which is the latest to hit the Nats centre at Swanwick in Hampshire.
The problem began around in mid-afternoon. To start with Nats said: "We can confirm that a technical problem has been reported at Swanwick air traffic control centre. We apologise for any delays and our incident response team has been mobilised.
"Every possible action is being taken to assist in resolving the situation and to confirm the details. Further information will be released as it becomes available."
Air traffic management company Eurocontrol said there had been "a failure" at the centre and airports would not be able to accept any traffic until 6pm UK time.
Nats said: "We can confirm that a technical problem has been reported at Swanwick air traffic control centre. We apologise for any delays and our incident response team has been mobilised.
"Every possible action is being taken to assist in resolving the situation and to confirm the details. Further information will be released as it becomes available."
Press Association sports reporter Simon Peach landed this afternoon at Heathrow after visiting Istanbul to cover Arsenal's Champions League tie against Galatasaray.
He said that passengers were stuck on his plane because their gate was being blocked by a grounded aircraft.
He said: "Pretty much as soon as we touched down the pilot came over (the radio) and said 'I'm really sorry about this but I have just been informed that no planes are departing from the south of England'.
"He said that as a result of the fact that the gate we were going to go in has a plane there we were to expect a considerable delay.
"At the moment we are just taxiing aimlessly as far as I can tell."
Heathrow said there were no aircraft departing or arriving at the west London airport.
It became clear that all London airspace would be closed until at least 7pm.
One passenger caught up in the travel chaos was Matt Warren.
He tweeted: "Stuck on the tarmac at Heathrow airport. Air traffic control failures. No flights in or out. "
A Birmingham Airport spokeswoman said the airport remained open. She went on:: "In most cases flights taking off to the north are not affected. Flights going south, some are delayed while some are re-routing."
She advised passengers to check the status of individual flights.
The state-of-the art centre at Swanwick has been subject to a number of computer glitches since Nats moved there from its old headquarters in West Drayton in west London in the early part of the last decade.
One of the worst problems was a year ago - on Saturday December 7 2013 - when thousands of passengers were left stranded when hundreds of flights were grounded following a technical fault at the Hampshire centre.