Heathrow Airport is thinking about taking legal action against National Grid following a new report that shed light on the cause of a fire earlier this year that grounded flights.
In March 2025, thousands of passengers were stranded at Heathrow and other airports worldwide after a nearby substation fire left the London airport without electricity.
A review into a fire that led to widespread chaos has found that National Grid failed to maintain its infrastructure, resulting in the blaze of a substation in Hayes, which it managed.
More than 270,000 passengers were impacted by the disaster, with planes grounded and airlines reportedly losing millions of pounds due to the incident, which is thought to be the worst delay experienced at the airport since 2010 when snow forced it to close.
Now, a report by the National Energy System Operator (NESO) has laid bare more details about what exactly went wrong. It’s also prompted Ofgem to launch its own investigation into what it believes was a “preventable technical fault” in the National Grid.
So what do we know?
What caused the Heathrow delays?
On March 21, a fire at an electrical substation several miles away from London’s busiest terminals led to power being cut across the airport.
Reports at the time claim that 70 firefighters rushed to the scene of the fire, which took several hours to be safely extinguished. It wasn’t clear initially what had caused the fire, but experts later ruled out foul play.
However, it led to Heathrow remaining closed throughout Friday, impacting hundreds of thousands of passengers and leaving thousands of tonnes of cargo stranded.
At the time, Heathrow's CEO Thomas Woldbye reportedly turned his phone on silent and went to bed on the night of the shutdown, a move that he was later heavily criticised for.
Although some flights started to resume late on the Friday, regular services didn’t fully resume until around two days after the incident. This prompted initial concerns about how resilient airport operations were to withstanding such incidents in the future.
Heathrow report unveiled
According to NESO, the fire was caused by a “catastrophic” failure of a crucial electricity substation component that was overseen by the National Grid. The report revealed this was linked to moisture entering the substation component, which is believed to have been preventable.
“This review has seen evidence that a catastrophic failure on one of the transformer’s high voltage bushings at National Grid Electricity Transmission’s 275kV substation in Hayes caused the transformer to catch fire,” Wednesday’s review revealed.
“This was most likely caused by moisture entering the bushing, causing an electrical fault.” The report suggests that the electricity may have “arced” and triggered an ignition in the oil, which prompted the wider fire.
Findings also suggest that moisture had been detected in oil samples as early as 2018, “but mitigating actions appropriate to its severity were not implemented.”
Following the release of the report, 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.”
According to Energy Secretary Ed Miliband: “The report is deeply concerning, because known risks were not addressed by the National Grid Electricity Transmission, and Ofgem has now opened an official enforcement investigation to consider any possible licence breaches relating to the development and maintenance of its electricity system at North Hyde.
What’s the problem?
Heathrow bosses came under fire following the incident, largely over questions about the airport’s energy resilience.
Some experts had reportedly warned airport bosses prior to the incident that such risks existed.
There were also concerns about how an airport such as Heathrow could respond if there was a national security incident in the future.
“If one fire can shut down Heathrow’s primary systems ... it tells you something’s badly wrong with our system of management of such disasters,” Alan Mendoza, the executive director of the Henry Jackson Society, told AP at the time.
