LONDON, July 2 (Xinhua) -- The Office of Gas and Electricity Markets (Ofgem) launched an official enforcement investigation into National Grid Electricity Transmission on Wednesday, following the North Hyde Substation fire, which caused a major power outage that shut down Heathrow Airport in March.

A report published by the National Energy System Operator (NESO) on Monday has found that the National Grid missed two chances to prevent the catastrophic equipment failure at a nearby substation.

First, in 2018, when elevated moisture levels were detected in oil samples from a key component, and another in 2022, when essential maintenance was delayed. Experts say the fire at North Hyde Substation was likely triggered by moisture entering a high-voltage electrical insulator known as a bushing.

The bushing led to an explosion and fire that knocked out power to the airport, disrupting more than 270,000 passenger journeys and cancelling around 1,300 flights.

According to NESO, the National Grid failed to act on warning signs. Despite identifying a fault, no steps were taken to remove or repair the damaged transformer. Fintan Slye, NESO chief executive, said, "They identified a fault (but) for some reason the transformer didn't immediately get pulled out of service and get repaired."

The report added that the design of Heathrow's internal power network -- the loss of just one of three external supply points -- led to a total blackout of operationally critical systems, suspending airport functions for approximately 16 hours.

Also on Wednesday, Energy Secretary Ed Miliband said on social media X that the report was "deeply concerning," stating that "known risks were not addressed" by National Grid Electricity Transmission, which is affiliated with the National Grid.

Police have ruled out foul play in March and said that there was no evidence of suspicious activity. Enditem