Britain's Heathrow said on Friday it was unclear when Europe's busiest airport would be able to re-open after a fire knocked out its power, stranding passengers and angering airlines who questioned how such crucial infrastructure could fail.

Huge orange flames and plumes of black smoke shot into the sky around 2300 GMT on Thursday as a blaze engulfed a substation near the airport, cutting the power supply and a back-up system.

Police said that while there was no indication of foul play, they retained an open mind and counter-terrorism officers would lead the inquiries, given their capabilities and the critical nature of the infrastructure.

Heathrow, which is also the world's fifth-busiest airport, said at 1330 GMT that it did not have clarity on when power would be reliably restored, having previously said the airport would be shut until midnight.

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

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.

The fire brigade said the cause of the fire was not known, but that 25,000 litres of cooling oil in the substation's transformer had caught fire. By the morning the transformer could be seen smouldering, doused in white firefighting foam.

Also Read: Heathrow airport shuts down suddenly – Here’s latest update & schedule

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 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.

"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."

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 jumped, with booking sites offering rooms for 500 pounds ($645), 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. "We apologise for the inconvenience."

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.

Pictures on social media showed the airport terminals in near darkness during the night, and British energy minister Ed Miliband said it appeared that the "catastrophic" fire had prevented the power back-up system from working.

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.

"How is it that critical infrastructure - of national and global importance - is totally dependent on a single power source without an alternative," he said. "If that is the case - as it seems - then it is a clear planning failure by the airport."

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.

Experts in power supply said the type of fire that erupted overnight was extremely rare, but they added that there should be sufficient alternative supplies to get everyone back online quickly.

The National Grid said at 1400 GMT that it had reconfigured the network to restore power but only on an interim basis, and it was carrying out further work to return to normal operations.

Prime Minister Keir Starmer said he was receiving regular updates on the incident.

Midnight flight scramble

As the scale of the outage became clear, flights operated by airlines including 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.

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, D.C. rather than London.

"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, including U.S. carriers. EasyJet and Ryanair put on extra flights or used larger planes to help tackle the backlog.

The airport's owners include France's Ardian, the Qatar Investment Authority and Saudi Arabia's Public Investment Fund, plus other investors in Australia, China and Spain.

Businesses in the area around the airport include data centres and logistics warehouses.