Cause of Heathrow shutdown fire remains unknown, says system operator

Firefighters douse smoke at the North Hyde substation that supplies electricity to Heathrow airport. Photograph: Leon Neal/Getty Images

Investigators have failed to find the root cause of the substation fire that shut Heathrow airport in March, six weeks after the government ordered an “urgent” investigation.

A preliminary report from the National Energy System Operator (Neso) ruled out any suspicious activity behind the outage which cut power to the airport, affecting more than 1,350 flights and almost 300,000 passengers.