An inquiry has found that an electrical substation fire that shut down Heathrow Airport in March was caused by a preventable technical fault identified almost seven years earlier.

A fire that shut Heathrow airport in March, stranding thousands of passengers, was caused by National Grid's failure to maintain an electricity substation, an official report said on Wednesday, prompting the energy watchdog to open a probe.

National Grid blamed for failing to act on known transformer problem, while Heathrow accepted risk as a ‘high-impact, low-probability event’