Heathrow airport. Flickr/Tony Hisgett London's airspace was in meltdown on Friday as all its airports suspended flights due to a major technical problem.

Airspace reopened at around 4.30 p.m GMT, after around an hour of being closed, but serious delays were expected to continue into the evening due to knock-on effects.

"Flights are experiencing severe disruption due to a technical problem at the air traffic control centre. If you are due to depart today, please check your flight status before travelling to the airport," Heathrow said on its website.

The news broke on Twitter a little after 3 p.m. GMT as passengers at Heathrow, the UK's biggest airport, tweeted about suspended travel.

Heathrow Airport said initially that UK flights were experiencing delays due to a power outage at the NATS air traffic control centre in Swanwick.

However, it was then confirmed that all air traffic had been grounded — originally until at least 7 p.m GMT — due to a major computer failure. It caused major disruption to Heathrow, Gatwick, Stansted, and air travel globally.

Aberdeen Airport reported that the computer failure had been rectified before the 7 p.m estimate. At around 4.30 p.m, NATS said airports were functioning but disruption to times would carry on. NATS added that air traffic volumes would be restricted for some time, while Heathrow announced that flights were leaving and arriving, but advised passengers to check their bookings before travelling.

Here are some reactions on Twitter:

"NATIONAL AIR TRAFFIC COMPUTER SYSTEM HAS SUFFERED A FAILURE.UNTIL THE SYSTEM RECOVERS, NO TRAFFIC UNTIL 1900 UTC INITIALLY." — Lucy Tobin (@lucytobin) December 12, 2014

No departures out of @HeathrowAirport right now. RT @ThorpeDave: @AirlineFlyer Traffic going into heathrow but not out at the moment — Jason Rabinowitz (@AirlineFlyer) December 12, 2014

Pilot on my flight just announced "major computer failure at #London Heathrow's Air Traffic Control" all flights in/out have been suspended. — Nicholas J. Percoco (@c7five) December 12, 2014

Some people made the most of the situation:

Stuck on the runway at Heathrow en route to Berlin... but we have beer! And it's Christmas! @BBCLondonNews #Heathrow pic.twitter.com/cES0EHPuCo — Phil Damerell (@Phil_Damerell) December 12, 2014

This is what London's airspace looked like as of 3.30pm:

NATS initially released this information on the power failure:

"NATS 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."

Here's a video posted by someone at Heathrow during the melee:

The Network Operations Portal also reported a technical failure. It said engineers were immediately working to fix the problem.

Heathrow is the third busiest airport in the world. It handles 134,000 passengers per day. Gatwick is the 37th busiest, handling 96,000 passengers per day.

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