Flights began landing at Heathrow on Saturday morning as it aimed to return to normal operation after shutting down over a loss of power. Restrictions on overnight flights were temporarily lifted to help ease congestion, the Department of Transport said, and the airport said passengers should come to Heathrow on Saturday as they normally would.

British Airways, which has a major presence at Heathrow, said it expected to operate around 85% of its scheduled flights at the airport on Saturday. The airline would usually expect to run nearly 600 departures and arrivals on a Saturday but it is understood cancellations will be made, where possible, to high-frequency routes.

According to flight tracking website FlightRadar24, British Airways (BA) flight BA56 from Johannesburg, South Africa was the first regular passenger flight to land at Heathrow since Thursday evening, touching down at 4.37am on Saturday.

Downing Street said there were “questions to answer” after a fire at an electrical substation closed down the airport, stopping more than 1,350 flights and disrupting the journeys of hundreds of thousands of passengers.

Counter-terrorism police were leading the investigation into the “unprecedented” incident that left Britain’s biggest airport unable to function as engineers tried to restore power, but said there was “no indication of foul play”.

Two people familiar with the investigation said officials did not believe the fire was the result of any criminal activity or a hostile state act and was more likely to be accidental. Police confirmed on Friday evening that the fire was not thought to be suspicious.

Heathrow partially reopened on Friday evening but airlines said the closure would continue to have a “huge impact” on passengers in the coming days.

Ministers acknowledged the “immense distress and disruption” caused to passengers, vowing “we will learn the lessons”.

Asked about an inquiry and whether National Grid had questions to answer, the prime minister’s official spokesperson said: “There are questions to answer on how this has happened and what can be done to prevent the scale of disruption we’ve seen from happening again, once the situation is under control.”

Passengers were warned to stay away from Heathrow all day on Friday after the London fire brigade (LFB) was called to tackle the blaze that started shortly after 11pm on Thursday at a substation in Hayes, north of the airport.

More than 70 firefighters worked in “challenging and hazardous” conditions to bring the fire under control, the LFB said, but it was still burning at 8am on Friday, with 25,000 litres of oil having caught fire at the substation. Officers said that despite the acrid smell, there was no air quality danger to the public.

Power to 67,000 homes was cut off for several hours and more than 100 people were evacuated. Power supplies have now been restored to all customers, including Heathrow, allowing operations to resume at the airport.

More than 1,350 flights carrying at least 200,000 passengers had been scheduled to arrive and depart from the airport on Friday, connecting to around 230 destinations, including those in the US for which the airport acts as the main gateway from Europe.

About 120 long-haul flights to Heathrow were in the air when the closure was announced overnight, with arrivals diverted to other airports around Britain and Europe.

London residents will be disturbed by more aircraft noise at night than normally permitted in the coming days after the Department for Transport lifted restrictions on night flights to ease the backlog.

Heathrow has experienced significant disruption from external events, such as the air traffic control outage in August 2023 and the mass groundings during the Covid crisis, but the last time the airport was fully closed for an extended period was during heavy snowfall and freezing conditions in December 2010, when 4,000 flights were cancelled.

Senior security and aviation figures expressed consternation that the world’s best connected airport could be closed for so long because of a power cut.

The International Air Transport Association (Iata) said it was “yet another case of Heathrow letting down both travellers and airlines”. Willie Walsh, Iata’s director general, added: “How is it that critical infrastructure is totally dependent on a single power source without an alternative? If that is the case then it is a clear planning failure by the airport.”

David Omand, the former head of the Government Communication Headquarters (GCHQ), told the BBC’s World at One: “Given the importance of Heathrow I am surprised that the whole airport had to be shut for a day.

“I mean, you could understand disruption whilst you change over to alternate systems and so on, but such a complete failure over the period of a day – and who knows the disruption may last longer – is a national embarrassment. It shouldn’t have happened.”

However, the transport secretary, Heidi Alexander, defended the airport on Friday night, saying the “unprecedented situation” had been “totally outside of Heathrow’s control”.

“They have stood up their resilience plan swiftly, and they’ve collaborated closely with our emergency responders and the airline operators,” she said. “They do have backup energy supplies, they have generators, diesel generators.

“None of that failed on this occasion because that backup supply is designed to protect the critical key systems within the airport and not to provide power to the whole airport.

“I would still advise anyone who has got a flight tomorrow [Saturday] to check in with their airline before travelling to the airport. But given the scale and magnitude of this incident, the response has been swift, although I do appreciate there will have been immense distress and disruption to a very large number of people.”

Heathrow’s chief executive, Thomas Woldbye, apologised to “the many people who have had their travel affected”. But added: “I’d like to stress that this has been an incident of major severity. It’s not a small fire.”

The Metropolitan police said it was working with the LFB to establish the cause of the fire, which remains under investigation. A spokesperson said: “While there is currently no indication of foul play we retain an open mind at this time.”

Detectives were preparing to take away parts of the substation and its equipment to examine them for signs of interference. It is understood initial checks of CCTV covering the perimeter of the substation showed nothing suspicious.

Counter-terrorism detectives were expected to stay on the case until an alternative explanation emerged for the fire.

Alice Delahunty, the president of National Grid’s transmission business, said it had been “a very significant and serious incident, which is extraordinarily rare in our network” and that it was not possible to verify any “rumours and speculation” over the cause of the fire.

She added: “We’re not in a position to rule anything in or out. Our focus has been getting homes and businesses back on supply safely. There will be a time for a full and thorough investigation, but the focus has been on restoring power.”

A Heathrow spokesperson said it would initially work with airlines on repatriating the passengers who were diverted to other airports in Europe.

“Our priority remains the safety of our passengers and those working at the airport. As the busiest airport in Europe, Heathrow uses as much energy as a small city, therefore getting back to a full and safe operation takes time. We apologise for the inconvenience caused by this incident.”

British Airways, which operates around half of all Heathrow flights, resumed long-haul services on Friday evening after power was restored. Flights also began arriving at the airport.

The chief executive, Sean Doyle, said BA had “been forced to effectively ground our flying operation”, cancelling every short-haul and the majority of long-haul flights scheduled for Friday.

“Unfortunately, it will have a huge impact on all of our customers flying with us over the coming days,” he said.