The fire at an electrical substation that forced London’s Heathrow Airport to close for more than 12 hours this year was caused by a short circuit in a transformer part that did not get needed maintenance for nearly eight years, British government regulators said in a report released early Wednesday.

More than 1,000 flights were disrupted at Heathrow, one of the world’s busiest travel hubs, during the March 21 shutdown after the fire cut electricity to much of the airport, disabling critical safety systems and forcing officials to stop all incoming and outgoing flights for most of the day.

Officials had previously said that the outage was caused by a spectacular fire at the nearby substation and had largely ruled out terrorism or other foul play. But until Tuesday, officials had not said how the fire started in the first place.

The 77-page final report found that problems with a bushing — an insulated conductor in the transformer — had been detected in 2018 but not resolved. Instead, officials said, the issue went untreated for years, including when maintenance on the transformer was deferred in 2022.

They added that basic maintenance on the transformer, called SGT3, was last performed by National Grid Electricity Transmission, the company that operates the substation, in July 2018.

“Evidence provided to the review revealed that multiple attempts were made to schedule basic maintenance on SGT3, none of which went ahead,” the report said.

The loss of power at Heathrow prompted questions from travelers, airline officials and some of Britain’s political leaders about the vulnerability of the country’s electrical grid. Prime Minister Keir Starmer said at the time that he was “deeply concerned” about the airport’s closure, adding that there were “questions that need to be answered in relation to what happened.”

The report released on Wednesday also concluded that Heathrow’s internal power systems, which were disrupted by the fire, were not designed to quickly take advantage of power from two other nearby substations that were not affected. The report said that airport officials had not expected to have to deal with a catastrophic power outage.

“The loss of supplies from a supply point was not assessed to be a likely scenario” by the airport company, the report said, “due to its expectation of the resilience of the wider network.”

The report added that the airport’s “internal electrical distribution network was not designed or configured to take advantage of having multiple supply points to provide quick recovery following such a loss and relied on manual switching.”