The root cause of the fire that sparked a major power outage at Heathrow Airport remains unknown, the UK's national energy system operator (Neso) has said. It added that power was restored to Heathrow airport’s terminals around seven hours before flights resumed on the day it was closed because of a substation fire.

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.

Energy Secretary Ed Miliband has welcomed the interim report by Neso into the Heathrow airport power outage. He said: “My department took immediate action to investigate the power loss, which impacted Heathrow and the surrounding area, causing major disruption to thousands of people and many businesses.

“The National Energy System Operator’s initial summary of 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.”

Neso, in its interim findings into the North Hyde Substation outage, said the London Fire Brigade and National Grid Electricity Transmission were continuing to conduct forensic investigations.

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 findings found the flow of electricity to all four of the west London airport’s passenger terminals was restarted by 10.56am on March 21.

Flights did not resume until approximately 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”.

Heathrow airport said the report 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 spokesman said: “Heathrow welcomes the Neso review’s initial report, which raises important questions for National Grid and SSEN that we hope the final report will provide answers to, including the cause of the fire.

“Further clarity on how the fire started and why two transformers were subsequently impacted can help ensure greater resilience for the UK’s energy grid moving forward.”