Heathrow Airport is considering legal action against National Grid after a report found the fire which caused the airport to shut down was a result of known fault at an electrical substation.

An investigation found that National Grid, which owns the substation which supplies Heathrow, had been aware of a problem since 2018 but failed to fix it.

There were numerous opportunities to rectify moisture affecting electrical parts at the North Hyde substation, but maintenance was repeatedly deferred, the report said.

Heathrow told the BBC National Grid “could and should” have prevented the fire and that it expected it to “take accountability for those failings”.

“Those failings that resulted in significant damage and loss for Heathrow and our airlines,” a spokesperson added.

National Grid said it had taken action since the fire on 20 March, but said such events were “rare” and that Britain had “one of the most reliable networks in the world”. It has not yet responded to Heathrow’s potential legal case.

Following the report being released on Wednesday, energy watchdog Ofgem has launched its own investigation into National Grid.

Heathrow, the UK’s biggest airport, shut down as a result of the power cut, which led to thousands of cancelled flights and stranded passengers.

The National Energy System Operator (Neso) said moisture entering electrical components at the substation caused the blaze.

It said “elevated” moisture had been detected in July 2018 and that under National Grid’s guidance, such readings indicate “an imminent fault and that the bushing should be replaced”.

Bushing is insulating material used around electrical parts.

But it said the issue went “unaddressed” and an “basic maintenance” to fix the problem in 2022 was deferred.

More than 270,000 journeys were affected by Heathrow’s shut down and the impact was also felt beyond the airport, with “essential services” including road, rail and Hillingdon Hospital affected, Neso said.

Airlines based at Heathrow have said the closure on 21 March cost carriers between £80m to £100m.

The power failure and subsequent closure of the UK’s busiest airport has raised wider questions about the UK’s major infrastructure resilience.

Fintan Slye, Neso’s chief executive, told the BBC’s Today programme that while the report “did not set out to apportion blame”, National Grid was responsible for the maintenance of the equipment and making sure it is fit for operation.

The report found that National Grid did not know how crucial the North Hyde substation was to Heathrow, even though the airport did know.

“So when they identified those moisture levels back in 2018, what should have happened is that the transformer should have been taken out of service for a short period of time and that fault effectively repaired,” he said.

“That was missed. The moisture got worse over time and ultimately that’s what caused the fire to happen. The story of our report really is probably one of missed opportunities.”

After announcing its investigation into National Grid, Akshay Kaul, director general for infrastructure at Ofgem, said it expected energy companies to “properly maintain their equipment and networks to prevent events like this happening”.

“Where there is evidence that they have not, we will take action and hold companies fully to account,” he added.

National Grid said it had a “comprehensive asset inspection and maintenance programme in place”.

It said this included “re-testing the resilience of substations that serve strategic infrastructure” and added it would cooperate with Ofgem’s investigation.

Mr Slye said as well as the substation fault, there was also a “potential missed opportunity” at Heathrow to maximise its energy resilience and also another to across the energy industry around understanding resilience of critical infrastructure.

The report also said that “it was not known to the energy companies” that the loss of one of the electrical supply points – of which Heathrow has three – would result in a power outage to some of the airport’s critical systems.

“The review also found that energy network operators are not generally aware whether customers connected to their networks are Critical National Infrastructure,” it said.