Power was restored to Heathrow Airport's terminals seven hours before flights re-started on the day it was closed because of a substation fire, an investigation has found. An interim report by the National Energy System Operator (NESO) found the flow of electricity to all four of the west London airport's passenger terminals was restarted by 10.56am on March 21. But 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".

More than 270,000 air passenger journeys were disrupted by the closure. The fire at the nearby North Hyde substation started late the previous evening. Its cause remains unknown.

The report stated that one of 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 March 20. 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.

The report said: "The consequence was the loss of all supplies from North Hyde 275kV substation, impacting thousands of customers including Heathrow airport."

Heathrow Airport said the report raises "important questions" for National Grid - which owns the substation - and Scottish and Southern Electricity Networks (SSEN), which is responsible for power distribution in the area.

A spokesperson 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."

Energy Secretary Ed Miliband, who commissioned the report, said: "My department took immediate action to investigate the power loss, which impacted Heathrow and the surrounding area (on March 20 and 21), 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."

The full report is expected to be published by the end of June.