LONDON (Reuters) -Flights at Britain's Heathrow resumed late on Friday after a fire knocked out its power supply and shut Europe's busiest airport for the day, stranding tens of thousands of passengers and causing travel turmoil worldwide.

Heathrow said its teams worked tirelessly to reopen the world's fifth-busiest airport after it was forced to close entirely after a huge fire engulfed a nearby substation on Thursday night, with travellers told to stay away.

The airport had been due to handle 1,351 flights on Friday, flying up to 291,000 passengers, but planes were diverted to other airports in Britain and across Europe, while many long-haul flights returned to their point of departure.

Heathrow said there would be a limited number of flights on Friday, mostly focused on relocating aircraft and bringing planes into London.

"Tomorrow morning, we expect to be back in full operation, to 100% operation as a normal day," said Heathrow chief executive Thomas Woldbye. "What I'd like to do is to apologise to the many people who have had their travel affected ... we are very sorry about all the inconvenience."

Police said that after an initial assessment they were not treating the incident as suspicious, although enquiries remained ongoing. London Fire Brigade said its investigations would focus on the electrical distribution equipment.

The closure not only caused misery for travellers but provoked anger from airlines, which questioned how such crucial infrastructure could fail.

The industry is now facing the prospect of a financial hit costing tens of millions of pounds, and a likely fight over who should pay.

"You would think they would have significant back-up power," one top executive from a European airline told Reuters.

Heathrow's Woldbye said back-up systems and procedures had worked as they should.

"This (power supply) is a bit of a weak point," he told reporters outside the airport. "But of course contingencies of certain sizes we cannot guard ourselves against 100% and this is one of them."

Asked who would pay, he said there were "procedures in place", adding "we don't have liabilities in place for incidents like this".

British transport minister Heidi Alexander said the incident had been out of Heathrow's control.

"They have stood up their resilience plans very swiftly and have been working in close collaboration with all the emergency responders and the airline operators," she told reporters.

DIVERTED

Airlines including JetBlue, American Airlines, Air Canada, Air India, Delta Air Lines, Qantas, United Airlines, IAG-owned British Airways and Virgin were diverted or returned to their origin airports in the middle of the night, according to data from flight analytics firm Cirium.

Shares in many airlines, including U.S. carriers, fell.

Aviation experts said the last time European airports experienced disruption on such a large scale was the 2010 Icelandic volcanic ash cloud that grounded some 100,000 flights.

While flights are restarting, it will be some time before all scheduled passenger services return to normal.

"We have flight and cabin crew colleagues and planes that are currently at locations where we weren't planning on them to be," said Sean Doyle, chief executive of British Airways, the biggest carrier at Heathrow which had 341 flights scheduled to land there on Friday.

"Unfortunately, it will have a huge impact on all of our customers flying with us over the coming days."

Britain's Department for Transport said it had temporarily lifted restrictions on overnight flights to ease congestion.

Passengers stranded in London and facing the prospect of days of disruptions were scrambling to make alternate travel arrangements.

"It's pretty stressful," Robyn Autry, 39, a professor, who had been due to fly home to New York. "I'm worried about how much is it going to cost me to fix this."

Prices at hotels around Heathrow jumped, with booking sites offering rooms for 500 pounds ($645), roughly five times the normal price levels.

A WAKE-UP CALL

Airline executives, electrical engineers and passengers questioned how Britain's gateway to the world could be forced to close by one fire, however large.

Heathrow and London's other major airports have been hit by other outages in recent years, most recently by an automated gate failure and an air traffic system meltdown, both in 2023.

Philip Ingram, a former intelligence officer in the British military, said Heathrow's inability to keep operating exposed vulnerability in Britain's critical national infrastructure.

"It is a wake-up call," he told Reuters. "There is no way that Heathrow should be taken out completely because of a failure in one power substation."

Willie Walsh, the head of the global airlines body IATA and a former head of British Airways, said Heathrow had once again let passengers down.

Heathrow said it had diesel generators and uninterruptible power supplies in place to land aircraft and evacuate passengers safely. Those systems all operated as expected. But with the airport consuming as much energy as a small city, it said it could not run all its operations safely on back-up systems.

Prime Minister Keir Starmer's spokesperson said there were questions to answer about how the incident occurred and there would be a thorough investigation.

(Reporting by Angela Christy, Surbhi Misra, Devika Madhusudhanan Nair and Gnaneshwar Rajan in Bengaluru, Jamie Freed in Sydney, Dan Catchpole in Seattle, Abhijit Ganapavaram in New Delhi, Joanna Plucinska, Muvija M, Catarina Demony, Andrew MacAskill in London, Tim Hepher in Paris, writing by Kate Holton and Michael Holden, Editing by Catherine Evans, Joe Bavier, Alison Williams, Kirsten Donovan and Daniel Wallis)