“It was a red-eye flight and I’d already had a full day, so I don’t even know how long I’ve been up for,” Hayes told the BBC as he was getting off the plane in Scotland. "Luckily I managed to get hold of my wife and she’s kindly booked me a train ticket to get back to Euston, but it’s going to be an incredibly long day.”

Heathrow is one of the world’s busiest airports for international travel. It had its busiest January on record earlier this year, with more than 6.3 million passengers, up more than 5% from the same period last year.

Still, the disruption Friday fell short of the one caused by the 2010 eruption of Iceland’s Eyjafjallajokull volcano, which spewed clouds of ash into the atmosphere and created trans-Atlantic air travel chaos for months.

Unclear what caused the fire but foul play isn't suspected

It was too early to determine what sparked the huge blaze about 2 miles (3 kilometers) from the airport, but there’s “no suggestion” of foul play, Energy Secretary Ed Miliband said.

The Metropolitan Police force said counterterrorism detectives were leading the investigation because of their ability to find the cause quickly and because of the location of the electrical substation fire and its impact on critical national infrastructure.

Heathrow said its backup power supply designed for emergencies worked as expected, but it was not enough to run the whole airport. It said it had no choice but to close the airport for the day.

“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.

The widespread impact of the fire that took seven hours to control led to criticism that Britain was ill prepared for disaster or some type of attack if a single blaze could shut down Europe’s busiest airport.

“The U.K.’s critical national infrastructure is not sufficiently hardened for anywhere near the level it would need to be at to give us confidence this won’t happen again,” said Alan Mendoza, the executive director of the Henry Jackson Society, a security think tank. “If one fire can shut down Heathrow ... it tells you something’s badly wrong with our system of management of such disasters.”

Tom Wells, a spokesperson for Prime Minister Keir Starmer, acknowledged that authorities had questions to answer and said a rigorous investigation was needed to make sure “this scale of disruption does not happen again.”

Heathrow — where the U.K. government plans to build a third runway — was at the heart of a shorter disruption in 2023 when Britain’s air traffic control system was hit by a breakdown that slowed takeoffs and landings across the U.K. on one of the busiest travel days of the year.

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