LONDON - Britain's Heathrow Airport was shut on Friday after a huge fire at a nearby substation knocked out its power, stranding passengers around the world and angering airlines who questioned how such crucial infrastructure could collapse.

Huge orange flames and plumes of black smoke shot into the sky around 11 pm (2300 GMT) on Thursday as a blaze engulfed the substation, cutting the power supply and back-up system for Europe's busiest and the world's fifth-busiest airport.

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

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The industry is now facing the prospect of a financial hit costing tens of millions of pounds.

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

The fire brigade said the cause of the fire was not known, but that 25,000 litres of cooling oil in the transformer had caught fire. It had brought the blaze under control by the early morning with the transformer doused in white firefighting foam.

Energy Minister Ed Miliband said it did not appear to be foul play.

Heathrow had been due to handle 1,351 flights on Friday, flying up to 291,000 passengers. The closure forced flights to divert to other airports in Britain and across Europe, while many long-haul flights simply returned to their point of departure.

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

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"When we first came here, (it was) very exciting and hopeful," said Beau Mahr, 21 from the US state of Iowa. "Now that we have to wait, it's kind of stressful."

Industry experts warned that some passengers forced to land in Europe may have to stay in transit lounges if they lack the paperwork to leave the airport.

Global flight schedules will also be affected, as aircraft and crews will now be out of position, forcing carriers to rapidly reconfigure their networks.

Prices at hotels around Heathrow also leapt, with booking sites offering rooms for 500 pounds ($646), roughly five times the normal price levels.

"Passengers are advised not to travel to the airport and should contact their airline for further information," Heathrow said, adding that the airport would be shut until midnight on Friday. "We apologize for the inconvenience."

Chaos ahead

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

Pictures on social media showed the airport terminal in near darkness during the night.

Experts in power supply said the type of fire that erupted overnight was extremely rare, adding that the whole substation likely had to be shut down to fight the blaze.

But they added that there should be sufficient alternative supplies to get everyone back on-line quickly.

"We can be fairly confident they will be able to restore by tomorrow," said Nicholas Rigby, a commercial engineer at NRG Management Consultancy.

Miliband said the "catastrophic" fire had prevented the power back-up system from working and that engineers were trying to deploy a third back-up mechanism.

"With any incident like this we will want to understand why it happened and what if any lessons it has for our infrastructure," he told Sky News.

At least 37 flights operated by airlines jetBlue, American Airlines, Air Canada, Air India, Delta Air, 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.

"Heathrow is one of the major hubs of the world," said Ian Petchenik, spokesman for flight tracking website FlightRadar24. "This is going to disrupt airlines' operations around the world."

British Airways, the biggest carrier at Heathrow, had 341 flights scheduled to land there on Friday.

Shares in its parent company IAG were hit, along with shares in other airlines. EasyJet and Ryanair put on extra flights or used larger planes to help tackle the backlog.

"It's going to be a chaotic couple of days," said travel industry analyst Henry Harteveldt with Atmosphere Research Group.

Qantas Airways sent its flight from Perth to Paris, a United Airlines New York flight headed to Shannon, Ireland, and a United Airlines flight from San Francisco was due to land in Washington, DC rather than London.

Some flights from the United States were turning around mid-air and returning to their point of departure.

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

A Heathrow spokesperson told Reuters in an email that there was no clarity on when power would be restored, and they expected significant disruption over the coming days.

On the ground in London, a number of homes and businesses were also without power.