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, and firefighters were still working to extinguish the blaze as of early Friday morning.

The brigade said it evacuated 150 people from the area. 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.

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 and located 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.”

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

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 wedding 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.

Meanwhile 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 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,” 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!”

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.