Workers on the way of investigate electrical substation following a fire at an electrical substation supplying power to the facility, in London, United Kingdom on March 21, 2025.
Rasid Necati Aslim | Anadolu | Getty Images
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.
The closure of Heathrow, the busiest airport in Europe, cost airlines tens of millions of pounds. It also raised questions about the resilience of Britain’s infrastructure.
Energy minister Ed Miliband said the report was “deeply concerning”.
The report follows a review of the incident on March 21 by the National Energy System Operator, which manages the electricity network and was separated from National Grid last year.
As a result of the report, Britain’s energy regulator Ofgem launched an enforcement investigation into National Grid Electricity Transmission, which owns and maintains the high-voltage electricity transmission network in England and Wales.
The report said its review had seen evidence that 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.
“This was most likely caused by moisture entering the bushing, causing an electrical fault,” it said.
It said National Grid controls in place were not effective and failed to identify that action had not been taken in relation to an elevated moisture reading in 2018.
A National Grid spokesperson said that the company had a comprehensive asset inspection and maintenance programme in place and that it had taken further action since the fire.
“There are important lessons to be learnt about cross-sector resilience and the need for increased coordination,” the spokesperson said.
Ofgem said the review had “established that the root cause of the fire was a preventable, technical fault”.
It plans to review whether National Grid complied with the relevant legislation and licence conditions relating to the development and maintenance of its electricity system close to Heathrow.
The regulator will also commission an independent audit into National Grid assets to review whether the failings were a one-off or systemic, it said.