By Todd Symons, Alex Stambaugh, Jessie Yeung and Sana Noor Haq

(CNN) — London’s Heathrow Airport has been brought to a complete standstill by a “significant power outage,” setting off waves of global travel chaos from one of the world’s busiest transit hubs.

Friday’s shutdown was caused by a fire in an electrical substation in the town of Hayes, just a few miles from the airport, which disrupted the local power supply, throwing more than 1,000 flights into disarray and forcing pilots to divert their journeys in midair.

More than 145,000 air passengers could be impacted by the closure, which officials warned could spiral into “significant disruption” over the coming days.

Arrivals into Heathrow were due to land from cities across the globe, including Sydney, Hong Kong, Bangkok, Singapore, Johannesburg, New York and Miami.

“We expect significant disruption over the coming days and passengers should not travel to the airport under any circumstances until the airport reopens,” Heathrow Airport said in a statement to CNN, adding that they “do not have clarity on when power may be reliably restored.”

Authorities have launched an investigation into the cause of the substation blaze, which is still burning but is now under control. So far, there are no signs of foul play, according to police.

Here’s what we know:

What happened?

A transformer at an electrical substation in the London suburb of Hayes caught fire Thursday night, according to the London Fire Brigade (LFB).

Ten trucks and around 70 firefighters were deployed to battle the blaze, the fire brigade said. A 200-meter (650-feet) cordon was set up around the scene.

“The fire in Hayes is now under control, but we will remain on scene throughout the day,” the LFB said in an update on X.

“As of the most recent update that I have received, approximately 10% remains alight” of the “high voltage substation,” LFB Deputy Commissioner Jonathan Smith also said in a news conference Friday morning.

“The fire involved a transformer comprising of 25,000 liters of cooling oil, fully alight. This created a major hazard due to the still-live, high-voltage equipment and the nature of the oil fuel fire,” Smith added.

Some 150 people were evacuated from the neighborhood, the brigade said. More than 16,000 homes lost power, according to utility supplier Scottish and Southern Electricity Networks — with Britain’s National Grid “working at speed” to restore power.

No injuries have been reported following the fire, the LFB confirmed Friday.

“As we head into the morning, disruption is expected to increase, and we urge people to avoid the area wherever possible,” LFB Assistant Commissioner Pat Goulbourne said in a statement.

In its statement to CNN, the airport said: “We know this will be disappointing for passengers and we want to reassure that we are working as hard as possible to resolve the situation.”

Heathrow Airport appeared largely dark amid the power outage, according to videos shared on social media. Terminal Two and Terminal Four are still out of power, LFB Commissioner Smith told reporters.

How did the fire start?

Investigations into how the blaze began are underway and London’s counter-terrorism police are now leading the probe, “given the impact this incident has had on critical national infrastructure.”

A police spokesperson said there was “no indication of foul play” but detectives “retain an open mind” over the cause.

Britain’s energy minister also said there was “no suggestion” of foul play. “At the moment, the focus is on restoring the power. We don’t yet have any… real understanding… of what caused the fire,” Ed Miliband told London’s LBC Radio.

“There’s no suggestion that there is foul play, just a catastrophic accident, is what we’re looking at,” he added.

The backup power supply at Heathrow was also hit by the fire, according to Miliband, leaving no proxy.

Earlier, an aviation analyst told CNN the apparent failure of the airport’s back-up electricity supply was “extraordinary.”

“You are our major hub to the world. You are incredibly important to the economy of the United Kingdom. There has to be a plan B,” said aviation analyst Geoffrey Thomas.

In an interview with the BBC, Miliband warned that lessons will have to be learned about “protection and the resilience that is in place for major institutions like Heathrow.”

How bad is the disruption?

Analysts forecast myriad supply challenges for airlines trying to reroute hundreds of flights to and from the London airport – a travel base that hosts a quarter of a million passengers and 1,300 flights from the United States, Southeast Asia and the Middle East every day.

Heathrow was the world’s fourth-busiest airport in 2023, according to the most recent data. Last year, a record-breaking 83.9 million passengers passed through. Spread across four terminals, it usually runs at 99% capacity, with every major airline crossing the hub.

Just on Friday, more than 1,350 flights going in or out of the airport will be affected, according to flight tracking website FlightRadar24. At the time of the closure, 120 airborne flights were immediately diverted to other airports or turned back to their original location.

The process to decide where the flights would be diverted is a dynamic and “fast-moving” one, according to aviation analyst Shukor Yusof.

“It requires a lot of coordination between the UK authority and the airline coming to land,” Yusof said, adding long-haul flights present “a more complex challenge” for rerouting.

Only a limited number of spots at nearby airports would be available, forcing airlines to potentially seek other options outside of the UK in this case, he explained. One important, potentially deciding factor in rerouting would be fuel, as pilots may also be asked to circle midair while waiting for a free runway to land on, he added.

Flights are being diverted all over Europe, including five flights to Manchester airport, about 200 miles northwest of London.

Ryanair added eight “rescue flights” between Dublin and London Stansted to help travelers. Air France suspended eight flights to and from Heathrow, but said routes to other UK airports were operating normally. Lufthansa canceled all flights to and from Heathrow on Friday.

British Airways and Qatar Airways said they were working with officials to update customers. Further afield, two Japanese flights that had already departed returned to Tokyo, and a third changed its destination, the Associated Press reported.

Trains around Heathrow – including the Heathrow Express, which connects the airport to central London – were also forced to stop services due to failed power supplies.

Business owners warned of significant economic fallout from the closure, after which shares in some airlines tumbled as much as 5%. Yusof, the founder of Singapore-based Endau Analytics, told CNN that the financial losses from the shutdown could be in the “hundreds of millions of pounds.”

What is happening to passengers?

Scenes emerged of tired travelers lining the corridors of Heathrow, while passengers in other airports spent hours stuck on the tarmac after their flights were cancelled.

Dual US-Norwegian citizen Kim Mikkel Skibrek had already been flying for three hours from Minneapolis to London when crew announced they had to turn back due to the fire.

“Everything is fine. People were frustrated with the flight returning after over three hours but now it seems everyone is calmer,” Skibrek told CNN. He was traveling home to Oslo to be with his father battling cancer, and said he will have to get another flight as soon as possible.

On the same flight, Abby Hertz and her family were traveling to London for the wedding of her husband’s best friend. The couple had postponed the ceremony once due to Covid and were finally getting married now that their son was in remission from leukemia, Hertz said — but it’s not clear if they’ll be able to make the wedding now.

At New York’s JFK Airport, passenger Christine said her British Airways flight had been ready to depart when the pilot announced they’d been asked to hold. Half an hour later, passengers were told Heathrow was closed and that another flight which had already taxied to the runway had been turned back.

“The mood is fairly relaxed on the plane, surprisingly. They’ve just come around to feed us,” said Christine, who declined to give her last name. But, she said, with a wedding in the UK to attend Saturday, “I really hope we’re not stuck until then!”

But for residents who live near the airport, birdsong has replaced the usual aircraft roar. “Basically living near Heathrow is noisy. There are planes every 90 seconds or so, plus the constant hum of traffic, but you get used to it, to the point of no longer noticing,” said James Henderson, who has lived next to Heathrow for over 20 years.

“Today is different. You can hear the birds singing,” he said.

This story has been updated with additional developments.

CNN’s Martin Goillandeau, Juliana Liu and Anna Cooban contributed to this report.

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