Mystery remains over huge fire that shut down Heathrow airport

The root cause of the fire that sparked a major power outage at Heathrow Airport remains unknown, the UK's national energy system operator has said.

The blaze in March at the North Hyde Substation, which supplies electricity to Heathrow about 1.5 miles away, caused a power outage, which meant Europe's biggest airport had "no choice but to close" for 16 hours. More than 1,000 flights to and from Heathrow were cancelled, and more than 270,000 air passenger journeys were disrupted.

Tens of millions of pounds were lost, thousands of passengers stranded and questions raised about the resilience of the UK's infrastructure. The National Energy System Operator (NESO), in its interim findings into the North Hyde Substation outage, said forensic investigations were ongoing.

The Metropolitan Police previously confirmed on 25 March that officers had "found no evidence to suggest that the incident was suspicious in nature". The interim report stated that one of North Hyde substation's three supergrid transformers - devices which enable voltages to be stepped up or down so electricity can be efficiently distributed - became disconnected, known as tripping, at 11.21pm on 20 March.

It was later confirmed to have caught fire. At 11.49pm, the adjacent transformer also tripped, which resulted in the simultaneous loss of connection to the remaining transformer, according to the document.

NESO anticipates that the final report, due to be published in June, is expected to make findings and recommendations relating to the resilience of energy infrastructure in the UK, the response and restoration of energy infrastructure, and the resilience of critical national infrastructure to energy disruption. Power was restored to the terminals around seven hours before flights resumed, investigators said, noting the flow of electricity to all four passenger terminals was restarted by 10.56am on 21 March.

The airport was shut for most of that day, before flights resumed around 6pm. NESO said power was restored to the "wider Heathrow Airport Limited network" by 2.23pm.

That was followed by "a period of safety checking" to ensure "safety critical systems were fully operational prior to passengers arriving at the airport". Read more on Sky News:'Learn from history', VE Day survivor urgesUS and UK 'agree trade deal'Alleged Aniston stalker charged Heathrow Airport said in a statement it welcomed the review which raises "important questions" for National Grid - which owns the substation that caught fire - and Scottish and Southern Electricity Networks (SSEN), which is responsible for power distribution in the area.

A spokesperson said understanding more about how the fire started and "why two transformers were subsequently impacted can help ensure greater resilience for the UK's energy grid moving forward". Energy Secretary Ed Miliband also welcomed the NESO report, admitting the fire and power loss led to "major disruption to thousands of people and many businesses".

He said the initial report into the incident "rules out the possibility of any suspicious activity. "We now await the full report to understand what happened and learn lessons to strengthen UK energy resilience and protect our critical national infrastructure." Heathrow is the world's fourth-busiest airport, handling 84.1 million passengers from March 2024 to February 2025..