The 35-minute shutdown caused flight delays in and out of London and flight slowdowns in other parts of Europe that officials said would linger into Saturday.
Some 66 flights were cancelled at Heathrow as of 1800 GMT (12:30 p.m. ET), with more expected, since planes and flight crews were out of position. Heathrow officials urged passengers to check the status of their flights before leaving for the airport.
NATS, the U.K-based air traffic management company, said a computer problem at its centre in Swanwick, England, had touched off troubles in the system. Heathrow officials blamed the situation on a power failure. After it was fixed, NATS said its operations were returning to normal but would not say how long that would take.
"We apologize for any delays and the inconvenience this may have caused," NATS said in a statement.
The shutdown, which began about 1527 GMT (10:27 a.m. ET) and ended at 1603 (11:03 am ET), came at the start of a busy weekend in a sprawling city with five commercial airports: London Heathrow — Europe's busiest — as well as Gatwick, Luton, Stansted and London City Airports.
Heathrow alone handles about 1,200 to 1,400 flights a day, some 200 of those to and from the United States, according to the flight tracking service FlightAware, which said Friday's problem most directly affected departures from London.
British aviation authorities did not ask the U.S. Federal Aviation Administration to hold flights destined for Heathrow on the ground during the shutdown, so there was no impact on departures from the U.S., said FAA spokeswoman Laura Brown.
Airline security analyst Chris Yates said when the airspace was closed, planes on the ground didn't take off because they would not be covered by any air traffic control. Those in the air close to the airport — say within 30 minutes of landing — were allowed to land.
The British government reacted with outrage to Friday's shutdown.
"Disruption on this scale is simply unacceptable and I have asked NATS for a full explanation of this evening's incident," Transport Secretary Patrick McLoughlin said.
The Swanwick centre has been plagued by problems since it opened in 2002 — six years after its planned commissioning date and at twice its original budget. Software and reliability issues have caused repeated disruptions since.
A few months after it opened, the BBC cited an unnamed air traffic controller as complaining of "potentially catastrophic" problems at the facility, including erratic radio transmissions. Other controllers cited by the broadcaster said the text on their computer screens was "too tiny to read."
In 2004, Swanwick was in the news again when a computer problem grounded scores of flights across Britain. An even more serious glitch in September 2008 grounded hundreds of flights and affected tens of thousands of travellers.
In December 2013, a computer problem at Swanwick took 12 hours to fix and caused travel chaos at the beginning of the holiday season. After calm was restored, NATS chief Richard Deakin called that failure a "one-in-10-year event."
Martyn Thomas, a visiting professor of software engineering at the University of Oxford, said while he didn't know the details of Friday's computer failure, he had examined NATS computer failures in the early 2000s.
"Some of NATS' computer systems are very old — the National Airspace System (NAS) that performs flight data processing is software that dates from the 1960s," he said in a statement. "Interfacing new systems to this old software can create difficulties."
Thomas said NATS has an outstanding safety record but once controllers lost their computer-based tools, delays were inevitable "because without the tools, the controllers cannot handle as many simultaneous aircraft."
At Heathrow on Friday, passengers on United Flight 941 to Newark were told about the computer problem as they boarded. United employees said the plane would be loaded and head toward a runway in the hopes that the computer problem would be fixed soon. Passengers appeared nonplussed and good-natured about the delay.
Federica Ceresa, 34-year-old from London who was returning home after a business trip to France, was scheduled to land about 3:30 p.m. — just about the time the computer issue began.
The pilot of her British Airways Flight 368 said at first that the plane would be going to Paris. Then he said they were going to Brussels. They were just about to land when the plane was re-routed back to London.
"There was clapping of hands," she said brightly.
Even though her 90-minute flight took about four hours, she wasn't too upset. She saw many passengers waiting to re-book flights as well as the eerie sight of an empty departure lounge. She felt quite lucky.
"Now I can go to Friday drinks!" she told an Associated Press reporter.
Raphael Satter and Kathleen Carroll in London, Shawn Pogatchnik in Dublin and Joan Lowy in Washington, D.C., contributed.