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The London airspace is recovering from a computer failure that caused a brief closure of the airspace, Eurocontrol said on Friday.
After about a 40 minutes of closure, UK air traffic control service provider NATS told CNBC that the system has been restored and that it is working towards resuming normal operations.
NATS said it is investigating the cause of the issue but confirmed that it was not a power outage, contrary to some reports.
The Federal Aviation Administration said it sees no impact so far for U.S. flights bound for London. The city has told the FAA that the situation should be resolved in about two hours.
London's Heathrow Airport said on its website that flights are "currently experiencing delays and cancellations."
"NATS systems are now working and stable but there are still some restrictions to airspace," the statement said.
Heathrow added that Friday's airspace issue would likely affect some services on Saturday "because aircraft and crew will now be out of position." The airport said it has extra staff on duty to help passengers.
An airport spokesperson told CNBC there have been 50 delays as a result of the computer error.
One year ago on December 7, more than 300 flights in the same area had to be cancelled because of an upgrade to the NATS communications system.
European budget airline Ryanair Holdings said in an emailed statement that "it's unacceptable that the NATS ATC system dropped for the second time in 12 months, particularly on a busy Friday in the run up to Christmas."
British Transport Secretary Patrick McLoughlin also told Reuters that the aviation disruption was "simply unacceptable."
Earlier, the UK control service had said on its website that UK airspace has not been closed but that "airspace capacity has been restricted in order to manage the situation."
The control service had said that "a technical problem has been reported at Swanwick air traffic control centre" and said its response team was on the case.
Heathrow had said earlier that the airspace closure was due to a power outage at the air traffic control center in Swanwick on the southern coast of England.
Swanwick handles 5,000 flights every 24 hours and controls the 200,000 square miles of airspace above England and Wales, according to BBC.
The airspace was expected to be closed until 19:00 UTC, or 2:00 p.m. ET, Eurocontrol said on its website.
Soon after the announcement of the London airspace closure, nearly all the flights over the city had landed, while the rest appeared on their way to Heathrow.
Located south of Central London, Gatwick Airport said flights are departing but that there are delays due to air traffic computer issues, Reuters said. Earlier, the airport had said no flights were departing and that flights were arriving.
Airports in Birmingham and Manchester, in central and northern England, said they were unaffected by the airspace closure and were ready to accept diverted flights, Reuters said.
British Airways told the news service that passengers who did not wish to travel on Friday could seek a full refund or postpone travel.
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CNBC's Louisa Bojesen and Reuters contributed to this report.