The cause of the devastating fire that triggered Heathrow Airport’s mass shutdown earlier this year remains unknown, an investigation has found.

The blaze, which broke out at the North Hyde electricity substation just 1.5 miles from Heathrow, caused a major power outage on March 20, grounding 1,000 flights and affecting hundreds of thousands of passengers. The airport was forced to shut down for 16 hours — its longest closure since volcanic ash grounded flights in 2010.

Despite a thorough review, the National Energy System Operator (Neso) has been unable to determine how the fire started. A Met Police investigation earlier ruled out foul play, confirming there was no evidence of suspicious activity.

Power was fully restored to Heathrow’s four terminals by 10.56am on March 21, but flights did not resume until around 6pm after extensive safety checks on critical systems.

The incident has sparked a wider review of national infrastructure resilience and substation design standards.

A Heathrow spokesperson welcomed the interim findings but called for more answers, saying:

Further clarity on how the fire started and why two transformers were subsequently impacted can help ensure greater resilience for the UK’s energy grid moving forward.

Energy Secretary Ed Miliband said the government is awaiting the full Neso report to better understand what went wrong and strengthen the UK’s critical infrastructure protections.

The full report is expected later this year.