Heathrow fire: Cause of blaze revealed as 270,000 flights affected in 24-hours of travel chaos

Despite the report admitting there were two missed chances to prevent the failure, Neso also said that no action was taken to replace electrical insulators, known as bushings.

The fire, which broke out March 21, resulted in Europe's biggest airport closing for around 16 hours.

Around 1,300 flights were cancelled, leaving at least 270,000 passengers facing travel disruption.

No flights operated at the London airport until around 6pm in the evening.

Neso's final report said there was a “catastrophic failure” in one of the transformers, “most likely caused by moisture entering the bushing causing a short circuit”, which ignited the oil.

It added: "An elevated moisture reading in one of SGT3’s (the transformer’s) bushings had been detected in oil samples taken in July 2018.

• Firefighters rush to tackle ship blaze as billows of smoke soar over Humber
• Huge fire rips through Britain's second largest retail park as smoke billows through the sky
• Inferno rips through £4.5m Grade-II listed mansion as firefighters rush to tackle blaze

"According to National Grid Electricity Transmission’s relevant guidance, such readings indicate ‘an imminent fault and that the bushing should be replaced’.

"While the reading was recorded in National Grid Electricity Transmission’s online system, the mitigations appropriate to its severity were not actioned.

"The controls in place were not effective and failed to identify subsequently that action had not been taken in relation to the elevated moisture reading.

"This includes an opportunity in 2022 when a decision was taken to defer basic maintenance on SGT3. The issue therefore went unaddressed."

Following Neso's report, energy regulator Ofgem confirmed it is now opening an investigation into the Heathrow fire.

An Ofgem spokesman said: "Based on today’s report and findings, Ofgem will examine the incident, its causes and take further action as necessary."

The Metropolitan Police had initially launched its own investigation into the substation fire.

However, Scotland Yard quickly confirmed that officers "found no evidence to suggest that the incident was suspicious in nature".

Responding to Neso's report, Heathrow Airport urged the National Grid to take action to prevent a similar incident occurring again.

"A combination of outdated regulation, inadequate safety mechanisms, and National Grid's failure to maintain its infrastructure led to this catastrophic power outage," a spokesman said.

"We expect National Grid to be carefully considering what steps they can take to ensure this isn't repeated.

"Our own Review, led by former Cabinet Minister Ruth Kelly, identified key areas for improvement and work is already underway to implement all 28 recommendations."

Energy Secretary Ed Miliband separately described the report's findings as "deeply concerning", also vowing to publish a response to Neso.

He said: “Following the North Hyde Substation fire which caused widespread disruption, the Government took rapid action to commission the National Energy System Operator (NESO) to investigate the incident.

“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.

“There are wider lessons to be learned from this incident. My department, working across Government, will urgently consider the findings and recommendations set out by NESO and publish a response to the report in due course.”