CNN  — 

London’s Heathrow Airport announced a complete shutdown all day Friday due to a “significant power outage” due to a large fire nearby, causing massive disruption to one of the world’s busiest travel hubs as flights were forced to turn back midair or divert to other locations.

“Due to a fire at an electrical substation supplying the airport, Heathrow is experiencing a significant power outage,” Heathrow Airport said in a statement on X. “To maintain the safety of our passengers and colleagues, Heathrow will be closed until 23h59 on 21 March.”

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

A transformer at an electrical substation in Hayes, a London suburb located just a few miles from the airport, caught fire Thursday night, according to the London Fire Brigade. The cause is not yet known.

Dozens of firefighters were on the scene overnight, with 150 people evacuated. More than 16,000 homes have lost power, according to utility supplier Scottish and Southern Electricity Networks.

Videos shared on social media showed huge flames and smoke rising into the air early Friday.

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

The shutdown could affect tens of thousands of travelers. Heathrow was the world’s fourth-busiest airport in 2023, according to the most recent data, with a record-breaking 83.9 million passengers passing through last year.

Spread across five terminals andlocated 14 miles west of central London, it usually runs at 99% capacity, with every major airline passing through, meaning it’s always very busy.

“Heathrow handles about a quarter of a million passengers a day. It does that with about 1,300 flights a day,” aviation analyst Geoffrey Thomas told CNN on Friday. “We’ve got literally hundreds of flights coming in from the United States, from Southeast Asia, the Middle East — they’re all in the air at the moment.”

Airline analytics firm Cirium estimated that “upwards of 145,000” passengers could be impacted.

By early Friday, the airport’s website showed multiple scheduled arrivals diverted or canceled, though others are still listed as “expected” arrivals.

British passenger Christine told CNN on Friday that she and her fellow passengers were about to take off from New York’s JFK airport when they heard the news.

Christine, who declined to give her last name but showed proof of travel, said her British Airways flight had been ready to depart when the pilot announced they’d been asked to hold for a while. Half an hour later, passengers were told Heathrow was closed and that another flight which had already taxied to the runway had turned back — leaving them stuck on the tarmac.

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

According to flight tracking website FlightRadar24, more than 1,350 flights going in or out of Heathrow on Friday will be affected. It also said 120 flights were in the air when the announcement came. They had to be diverted to other airports or turned back to their original location.

Thomas added that while shorter domestic flights might be able to turn back, that’s not an option for long-haul international flights. There are several other airports near London, including Gatwick Airport and Stansted Airport, but those are likely “at capacity,” meaning diverted flights have to go further to find an alternative place to land — like in Glasgow or Edinburgh, he said.

And that could pose another problem. Those other airports, some of them smaller and lower-cost than Heathrow, aren’t equipped to handle the sheer number of diverted passengers coming their way, he said.

As authorities race to contain the fire and navigate the fallout, they’ll also face tough questions, Thomas said, including why such a crucial travel and economic hub wasn’t able to tap into a backup power source.

This is a developing story and will be updated.

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