Heathrow Airport was shut down and thousands of homes were left without power on Friday after a fire at a nearby electric supply substation.

More than 1,300 flights have been cancelled, passengers are being told not to travel to the airport, and more than 150 people have been moved out of their homes.

So what do we know about what went wrong?

A fire at North Hyde substation in Hayes, west London, caused the outage.

Firefighters were called at 23.23 GMT on Thursday and the fire was under control by 06:28 on Friday.

The cause of the fire is not yet known. Counter terrorism police are leading an investigation looking into whether there was any foul play.

It is not yet clear why a fire at one substation shut down Heathrow, the UK’s busiest airport, completely.

The airport had multiple sources of power but the fire had affected a “particularly important bit,” a National Grid source told the BBC.

The substation’s back-up generator also appeared to have been “knocked out,” Energy Secretary Ed Miliband said.

Heathrow appears to have been reliant on one critical component and the National Grid distribution network was not set up to supply sufficient electricity in this situation.

A Heathrow source said these questions would be investigated, but at present, all of the airport’s efforts were directed towards reopening safely.

A source who designs data centres, which require a similar amount of electricity to Heathrow, told the BBC data centres have two supplies from the National Grid and stand-by generators.

They also have batteries to cover electricity needs until the generators can start.

Willie Walsh, the director general of the International Air Transport Association, which represents airlines, said the outage raised “serious questions”.

“Firstly, how is it that critical infrastructure – of national and global importance – is totally dependent on a single power source without an alternative? If that is the case – as it seems – then it is a clear planning failure by the airport,” he said.

The airport has back-up power for certain systems, but kickstarting alternative power supplies for the whole airport takes time, and the systems need to be checked to ensure they are working properly.

Heathrow uses as much energy as a small city, so it is not possible for it to have the back-up power by itself to run its operation safely.

A Heathrow source said its back up diesel generators and uninterruptable power supplies in place all operated as expected.

The problem lay with the National Grid, the source said, pointing out thousands of homes had been left without power, not just the airport.

There are two National Grid substations close to Heathrow: one at North Hyde, north of the airport, and one at Laleham, south of the airport, according to energy analysis firm Montel Group.

It appears that only the North Hyde substation is connected to Heathrow through the local distribution network, says Phil Hewitt, director at the firm.

Although Heathrow has its own biomass combined heat and power plant, it seems that couldn’t provide full back-up capacity.

“This potential lack of resilience at a critical national and international infrastructure site is worrying,” said Mr Hewitt. “An airport as large and as important as Heathrow should not be vulnerable to a single point of failure.”

Robin Potter, a research fellow at Chatham House, said Heathrow was one of only two UK airports – Gatwick is the other – that has any level of regulation around its resilience standards.

“These are actually the better airports in the UK for how their resilience is assessed and regulated,” he said.

In 2023, the National Infrastructure Commission recommended to the government that it should set standards for some key sectors of infrastructure such as telecoms, water, transport and energy by 2025.

It followed up with a further report at the end of last year detailing how the government could do that for those sectors.

“Those have effectively been on the government’s desk since October 2023,” he said.

Power should be back on “in hours” but it was too soon to say exactly when, a National Grid source said.

However, Heathrow said it didn’t have any “clarity on when power may be reliably restored”.

There will be “significant disruption over the coming days and passengers should not travel to the airport under any circumstances until the airport reopens”.

“We know this will be disappointing for passengers and we want to reassure [them] that we are working as hard as possible to resolve the situation,” it said.

Reporting by Tom Espiner, Theo Leggett, Ben King, Oliver Smith and Simon Jack.