LONDON (Reuters) - Britain's Heathrow Airport was shut on Friday after a huge fire at a nearby electrical substation wiped out its power, disrupting flight schedules around the world.

Around 70 firefighters were tackling the blaze in the west of London, which led to a mass power outage at Heathrow, Europe's busiest and the world's fifth-busiest airport, and also knocked out its back-up power system.

Huge orange flames and plumes of black smoke could be seen shooting into the sky. Around 150 people were evacuated from nearby buildings and thousands of properties were without power.

The fire brigade said the cause of the fire was not known.

"Our firefighters are working tirelessly to bring the fire in #Hayes under control," the service said in an update on X at 05:48 GMT. "Part of a transformer remains alight.

"As we head into the morning, disruption is expected to increase, and we urge people to avoid the area wherever possible."

Heathrow said the airport, which was due to handle 1,351 flights during the day, flying up to 291,000 passengers, would stay closed until midnight as it was experiencing a significant power outage.

"Passengers are advised not to travel to the airport and should contact their airline for further information," it said. "We apologise for the inconvenience."

The fire, which was reported just after 11 p.m. (2300 GMT) on Thursday, forced planes to divert to airports across Britain and Europe, while many long-haul flights simply returned to their point of departure.

Energy Minister Ed Miliband said the "catastrophic" fire had prevented the power back up system from working and that engineers were working to deploy a third back-up mechanism. He said it was too soon to say what had caused the blaze.

"There was a back-up generator but that was also affected by the fire which gives a sense of how unusual and unprecedented it was," he told Sky News.

"With any incident like this we will want to understand why it happened and what if any lessons it has for our infrastructure."

Industry experts warned that some passengers forced to land in Europe may have to stay in transit lounges if they lack the visa paperwork to leave the airport. Global flight schedules will also be affected more broadly as many aircraft will now be out of position.

"Heathrow is one of the major hubs of the world," said Ian Petchenik, spokesman for FlightRadar24. "This is going to disrupt airlines' operations around the world."

British Airways, the biggest carrier at Heathrow, had 341 flights scheduled to land at Heathrow on Friday.

CHAOTIC DAYS AHEAD

According to flight tracking website FlightRadar24, at least 120 inbound flights to Heathrow were having to divert to other airports in the early morning.

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.

Some flights from the U.S. were turning around mid-air and returning to their point of departure.

Police cordoned off the roads around the airport, local train routes were stopped and before 7 a.m. around 40 people were seen walking away from Terminal 5 with their luggage.

Travel experts said the disruption would extend far beyond Heathrow.

Airlines' carefully choreographed networks depend on airplanes and crews being in specific locations at specific times. Dozens of air carriers will have to hurriedly reconfigure their networks to move planes and crews around.

"The other question is, 'What will airlines do to deal with the backlog of passengers?'", said travel industry analyst Henry Harteveldt with Atmosphere Research Group. "It's going to be a chaotic couple of days."

Some passengers turned to social media. Adrian Spender, who works at British retailer Tesco, said in a post on X that he was on an Airbus A380 that had been headed for Heathrow.

"#Heathrow no idea where we are going yet. Currently over Austria."

Heathrow, and London's other major airports, have been hit by outages in the past in recent years, most recently by an automated gate failure and an air traffic system meltdown, both in 2023.

A Heathrow spokesperson told Reuters in an email that there was no clarity on when power would be restored, and they expected significant disruption over the coming days.

On the ground in London, a number of homes and businesses were without power. "Firefighters have led 29 people to safety from neighbouring properties, and as a precaution, a 200-metre cordon has been established, with around 150 people evacuated," the fire brigade said.