An investigation into the substation fire that led to the shutdown of Heathrow in March has found it was caused by a fault that was discovered – but not acted on – seven years earlier.

A final report into the North Hyde substation outage by the National Energy System Operator (NESO) said a “catastrophic failure” on one of the transformer’s high voltage bushings at National Grid Electricity Transmission’s 275kV substation caused the transformer to catch fire.

The review said the fire, which forced the grounding of 1,300 flights and disrupted up to 270,000 passengers, was “most likely” caused by moisture entering the bushing, which caused a short circuit.

NESO Chief Executive Officer Fintan Slye said: “The power outage and closure of Heathrow Airport were hugely disruptive and our report seeks to improve the way parties plan for and respond to these incidents, building on the underlying resilience of our energy system.”

Responding to the findings, a Heathrow spokesperson said: “A combination of outdated regulation, inadequate safety mechanisms, and National Grid’s failure to maintain its infrastructure led to this catastrophic power outage.

“We expect National Grid to be carefully considering what steps they can take to ensure this isn’t repeated.”