Energy Secretary Ed Miliband has ordered an urgent investigation into the power outage caused by a substation fire that shut Heathrow Airport on Friday.

The investigation, led by the National Energy System Operator (NESO), would build a “clear picture” of the incident and UK’s energy resilience more broadly to prevent it “from ever happening again”, the Department for Energy Security and Net Zero said.

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

The fire at the North Hyde substation in west London that supplies power to the airport led to thousands of cancelled flights on Friday and stranded passengers across the world.

On Sunday Heathrow said it would be operating “another full schedule” with more than 1,300 flights after more than 250,000 passengers passed through the airport on Saturday.

“We have welcomed the Government’s announcement of an investigation into the cause and response to the off-airport power outage and have launched a review, to be chaired by former Transport Secretary Ruth Kelly, of Heathrow’s response,” a statement from the airport added.

More than 63,000 homes lost power in the outage caused by the fire, according to energy supplier Scottish and Southern Electricity Networks. About 150 people were evacuated from surrounding properties.

The Met Police has said that counter-terrorism officers were “leading inquiries” but are not treating the incident as suspicious.

Miliband said he has commissioned the investigation to “understand any wider lessons to be learned on energy resilience for critical national infrastructure, both now and in the future”.

NESO, which operates Britain’s electricity grid, is expected to report to the power regulator Ofgem and the government in six weeks with its initial findings.

Nearly 1,400 flights were disrupted by the airport’s closure, according to air traffic website flightradar24.com. Around 120 flights were diverted elsewhere.

Separately, Heathrow has announced a review of the airport’s response to the power outage and its crisis management plans will be undertaken by former Transport Secretary Ruth Kelly.

The airport said it was “open and fully operational” on Saturday morning, but the chaos has raised questions about the resilience of the major transport hub.

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

On Saturday, more than 30 flights due to depart from the airport were cancelled and more than 15 were delayed, according to Heathrow’s live departure board.

It also showed that more than 70 flights expected to arrive at the airport had been cancelled, including from Doha, Riyadh, Dubai, Manchester, and Newcastle Upon Tyne.

Farah Rafeeq was due to travel with a friend from Heathrow to Cambodia on Friday to attend a friend’s wedding but the mass cancellation of flights has meant she will miss part of the ceremony.

The 24-year-old found an alternative flight from Gatwick Airport that will get them to Cambodia for Sunday afternoon in time for the wedding reception but said they had to pay “double the amount” for a the new flight.

In Miami, Charlotte and her family were heading to Heathrow on Thursday night when their American Airlines plane was “turned around mid-flight”, she said.

“I have been stranded in Miami with my infant one year-old and three year-old son since then,” she told the BBC.

She said the family had been “passed around from hotel to hotel”, but now hopes to be booked on a flight on Sunday.

An American Airlines spokesperson said it has been “working around the clock to get customers on their way” to Heathrow “as quickly and efficiently as possible”, adding that it was providing overnight accommodation to passengers affected by the airport’s closure.

Substations, operated by National Grid, are designed to produce, convert, and distribute electricity at suitable voltage levels. Heathrow uses three electricity substations, each with a backup.

There are also backup diesel generators, and uninterruptible battery-powered supplies which provide enough power to keep safety critical systems such as aircraft landing systems running.

However, when the fire broke out the substation, it was out of action, along with its backup.

Heathrow’s main fall-back was the two remaining substations, but the airport’s CEO, Thomas Woldbye, told the BBC that it “takes time” to “switch them”.

He said the incident was “not created at Heathrow Airport, it was created outside the airport and we had to deal with the consequences”.

In a statement released on Saturday evening, Mr Woldbye welcomed the investigation, saying that “we will support every effort to understand the causes and impacts of yesterday’s off-airport incident”.

Heathrow’s chairman Lord Paul Deighton said the airport regrets the disruption caused and hopes people affected “understand that the decision was made in order to prioritise the safety of our passengers and colleagues”.

He added that the airport is “committed to finding any potential learnings from this unprecedented incident” when its review is conducted.

Additional reporting by Kris Bramwell.