Energy Secretary Ed Miliband has ordered the National Energy System Operator to “urgently investigate” the power outage caused by a substation fire that shut Heathrow Airport on Friday.

He is working with Ofgem and using powers under the Energy Act to formally launch the grid operator’s investigation.

“We are determined to properly understand what happened and what lessons need to be learned,” Mr Miliband said.

“That is why, working with Ofgem, I have today commissioned the National Energy System Operator to carry out an investigation into this specific incident and to understand any wider lessons to be learned on energy resilience for critical national infrastructure, both now and in the future.

“The Government is determined to do everything it can to prevent a repeat of what happened at Heathrow.”

The National Energy System Operator (Neso) is expected to report to the Department for Energy Security and Net Zero and Ofgem with initial findings within six weeks.

Flights resumed at the west London airport on Friday evening and restrictions on overnight flights were temporarily lifted after hours of closure.

A blaze knocked out an electricity substation in Hayes the previous evening.

Counter-terrorism officers from the Metropolitan Police, who have been leading the investigation into the fire, are not treating it as suspicious but inquiries are ongoing.

Heathrow said it has added 50 slots to Saturday’s schedule to facilitate an extra 10,000 passengers travelling through the airport.

A spokesperson said: “We can confirm that Heathrow is open and fully operational today.

“Teams across the airport continue to do everything they can to support passengers impacted by yesterday’s outage at an off-airport power substation.

“We have hundreds of additional colleagues on hand in our terminals and we have added flights to today’s schedule to facilitate an extra 10,000 passengers travelling through the airport.

“Passengers travelling today should check with their airline for the latest information regarding their flight.”

The airport later added that all terminals and all car parks are open.

Transport Secretary Heidi Alexander said: “This incident caused significant disruption but Heathrow, National Grid, and our emergency services have worked swiftly to get people travelling again.

“Heathrow is a massive airport that uses the energy of a small city, so it’s imperative we identify how this power failure happened and learn from this to ensure a vital piece of national infrastructure remains strong.

“Whilst Heathrow is back to business, some disruption is expected over coming days as things get back to normal so I encourage anyone travelling to check with their airlines and plan their journeys.”

Akshay Kaul, director general for infrastructure at Ofgem, said: “We saw yesterday the huge disruption that comes when energy supply is disrupted, and it’s important we now understand how that happened.

“Households and businesses should be able to have confidence in the resilience of critical national infrastructure, and Ofgem will work with the government and others to ensure Neso’s review goes as far as possible to ensuring steps are put in place to avoid any repeat of an incident of this scale in the future.

“To the extent the review finds any breaches of standards or licence obligations, we will not hesitate to take action.”

Farah Rafeeq, 24, was due to travel with Singapore Airlines on Friday from Heathrow with her 32-year-old friend Niken Wulan, to another friend’s wedding in Cambodia on Sunday.

The mass cancellation of flights means they will now miss part of the ceremony.

They found an alternative flight from Gatwick airport with Turkish Airlines and Bangkok Airways that will get them to Cambodia for Sunday afternoon.

Ms Rafeeq, who works in climate project management, told the PA news agency from Gatwick on Saturday: “The last few hours have been nightmarish because it is one of our closest friends’ wedding and we have to travel for at least 20 hours to get there.

“We had to pay double the amount, between £600 and £700, for the new flight, and we had planned this trip for months and had hotels booked and are flying to South-east Asia after the wedding.”

An American tourist praised his airline’s and Heathrow’s “fantastic” response to the disruption.

Speaking at the airport, holidaymaker Tim Kolb, who travelled from Dallas, Texas, told PA: “I was just getting ready to board the plane on Thursday night, and the news actually started coming across on our phones quicker than on the (airport) desk.

“But American Airlines did a fabulous job – we didn’t have to wait in the line or anything. They got us our accommodation reservation.”

Mr Kolb, 55, added: “I thought I was going to be there (Dallas) delayed two days, but I went over yesterday. It was organised well, in fact, they had several planes leaving within an hour of each other to Heathrow.”

BA, which has a major presence at Heathrow, said it expects 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 Saturday but it is understood some cancellations will be made, where possible to high-frequency routes.

Heathrow chief executive Thomas Woldbye said a back-up transformer failed during the power outage, meaning systems had to be closed in accordance with safety procedures so that power supplies could be restructured from two remaining substations to restore enough electricity to power what is described as a “mid-sized city”.

He apologised to stranded passengers and defended the airport’s response to the situation, saying the incident is as “as big as it gets for our airport” and that “we cannot guard ourselves 100%”.

Thousands of homes were left without power and more than 100 people were evacuated after a transformer within the substation caught fire.

Heathrow is Europe’s largest airport, with more than 83.9 million passengers travelling through its terminals in 2024, and around 200,000 passengers have been affected by Friday’s closure.

This is believed to be the worst disruption at Heathrow since December 2010, when thousands of Christmas getaway passengers camped in the terminals because of widespread cancellations caused by snow.

In April of that year, air travel was grounded across Europe because of an ash cloud caused by an Icelandic volcanic eruption.