Flights have resumed at Heathrow Airport, a day after a fire at a nearby electrical substation shut down operations at one of the world's major transport hubs.

Heathrow, the UK's busiest airport, said on Saturday morning it was open and fully operational, however flight disruptions are expected to last days.

British Airways estimated 85% of its planned flights would run on Saturday, but with delays throughout. As of 07:00 GMT the majority of departures had left as expected but of arrivals nine of the first 20 flights scheduled to land were cancelled.

More than 1,300 flights were affected on Friday, tracking website Flightradar24 said, and passengers were told not to travel to the airport unless their airline advised them to.

National Grid said that an "interim solution" had been found to allow power to be restored to customers including Heathrow Airport, saying that the network had been "reconfigured to restore all customers impacted".

Firefighters worked throughout the day to bring the blaze at North Hyde substation in Hayes, west London, under control.

Here's what we know so far.

A fire at an electrical substation in west London, which supplies Heathrow, caused a major power outage at the airport, prompting its closure.

It is not yet known what caused the fire at the substation, but Transport Secretary Heidi Alexander said there was no suggestion of foul play as counter-terrorism police investigate.

"The counter-terrorism command has specific capabilities and capacities that mean that they are used to conducting investigations at pace," she said, adding that they were needed as the fire took place next to a critical piece of national infrastructure.

Emergency services were first called to Nestles Avenues in Hayes, west London at 23:23 GMT on Thursday.

Jonathan Smith, London Fire Brigade (LFB) deputy commissioner, said the fire at the electrical substation involved a transformer containing 25,000 litres of cooling oil.

The fire was "very visible and significant," he said. The LFB said the fire was under control by 06:28.

On Friday evening, the service said the fire was "believed to be non-suspicious" and the investigation will "focus on the electrical distribution equipment".

Commander Simon Messinger, who is leading the Metropolitan Police's response to the fire, said: "Various specialist investigators continue to examine the scene and it is expected to take some time before full assessments can be completed."

But he repeated that "at this stage, there remains no indication of any foul play".

The substation is about a mile and a half away from the airport.

Energy Secretary Ed Miliband told BBC Radio 4's Today programme the substation's back-up generator "appears to have been knocked out".

Videos shared on social media showed tall flames and plumes of smoke billowing from the substation overnight.

People living nearby were advised to keep doors and windows closed to avoid smoke inhalation.

With Heathrow being such a busy airport for passenger journeys and global trade, questions have been raised over whether it has back-up systems in the event of power cuts.

The BBC understands that Heathrow does have back-up power for its key systems, but kickstarting these alternative power supplies for the whole airport takes time.

A source said it was not possible to switch the power back on immediately.

A Heathrow source also told the BBC that they have "multiple sources" of energy at the airport – with diesel generators and "uninterruptable power supplies" in place.

They added that when the power outage happened the back-up systems "all operated as expected".

The systems, however, are not enough to run the whole airport – hence the decision to close it down.

And even once the power is back on, there are countless systems which need to be rebooted and checked to ensure they are working properly and are stable.

It is unclear why Heathrow's own back-up systems were not adequate to keep the airport running when one critical component of its energy supply was knocked out, and it is also unclear why the National Grid transmission network was not set up to supply sufficient electricity.

A Heathrow source said these questions would be investigated.

The first flights since the fire took off from and landed at Heathrow on Friday evening, with the airport initially saying it would prioritise repatriation and relocation of aircraft.

Heathrow said the airport was "open and fully operational" on Saturday morning, however a number of flights were cancelled and disruptions were expected to last days.

The closure had knock-on effects at many other airports, as airlines cancelled and diverted flights.

The airport has apologised for the disruption and has advised passengers to contact their airlines for further information.

Its helpline number is 020 8757 2700.

Late on Friday, the Department for Transport said on X that restrictions on overnight flights at Heathrow had been temporarily lifted to ease congestion.

Only 5,800 flights are allowed to take off or land at Heathrow between 23:30 and 06:00 each year, under government restrictions designed to limit how much noise the airport makes at night.

At least 1,351 flights to and from Heathrow were affected on Friday, Flightradar24 said, with some 120 affected aircraft already in the air when the closure was announced.

The Foreign Office has advised UK citizens who are abroad and require urgent assistance to contact their teams via an online query form.

Air Canada and United Airlines have announced they will be resuming some or all of their flights from Friday evening. Virgin Atlantic has said it hopes to operate "a near full schedule" on Saturday with limited cancellations, adding that it will continuously review flights.

Gatwick Airport told the BBC it was aware of the situation at Heathrow Airport and stands "ready to support as required".

Several of Australia's Qantas airline planes have been diverted from London to Paris, with other flights likely to be affected, it said.

British Airways has cancelled all its short-haul flights due to operate to and from the airport on Friday.

Some long-haul flights - including to Cape Town, Johannesburg, Singapore and Rio de Janeiro - were later given clearance to depart from Heathrow from 1900 GMT and the airline said it was reviewing the fire's implications for Saturday's schedule and beyond.

Cathay Pacific, Hong Kong's main airline cancelled all its flights to London on Friday.

The Heathrow Express railway service said it was running a reduced service from Paddington to Heathrow.

In addition to passengers expecting to fly, disruption has been caused to thousands of homes in west London, which have been left without power.

About 150 people had to be evacuated from surrounding properties.

More than 16,300 homes lost power in a large-scale outage caused by the fire, energy supplier Scottish and Southern Electricity Networks (SSEN) said.

"We're aware of a widespread power cut affecting many of our customers around the Hayes, Hounslow and surrounding areas," it added.

National Grid said on Friday afternoon the network had been "reconfigured to restore all customers impacted, including the ability to resupply the parts of Heathrow airport that are connected to North Hyde".

Meanwhile, two nurseries and four schools in Hillingdon - the London borough Heathrow Airport is located in - are shut today.

Hillingdon Council are assisting 12 people who were evacuated from their homes by the emergency services with hotel accommodation.

Bin collections will also be impacted on Friday, the council warned.

Bus routes in the Hillingdon area have been affected and the M4 is closed between junction three and four, while the Terminal Four spur roads are also closed.

No injuries from the fire have been reported.

Counter-terrorism officers from the Metropolitan Police are now leading the investigation into the fire because of the location of the substation and the "impact on critical national infrastructure".

It added there was currently no indication of foul play but it is keeping "an open mind".

London Fire Brigade said it was working closely with the Metropolitan Police.