Heathrow bosses were facing questions last night about how a small fire shut down one of the world’s biggest airports.

Tens of thousands of travellers are still stranded across the globe today after a blaze at an electricity substation cut power to the airport.

It resulted in unprecedented scenes, grounding flights for most of the day and exposing a major vulnerability in the country’s infrastructure.

Last night, a probe into the disaster was being led by counter-terrorism police, while Westminster sources blamed human error.

Nearly 300,000 passengers were affected by the meltdown, which led to the cancellation or delay of more than 1,350 flights. 

Some 120 transatlantic services were forced to turn round in mid-journey.

Passengers on flights from Singapore and Perth were diverted to Paris before taking buses to London.

Eight long-haul British Airways flights departed last night after facing delays criticized by analysts. London Heathrow, the busiest airport in Europe, received backlash for its failure to have backup power in place.

Willie Walsh, director-general of global airlines body IATA and a former head of British Airways, said Heathrow had once again let passengers down.

‘Firstly, how is it that critical infrastructure – of national and global importance – is totally dependent on a single power source without an alternative,’ he said.

‘If that is the case – as it seems – then it is a clear planning failure by the airport.’

As Heathrow shut down sparking worldwide travel carnage: 

Phil Hewitt, director of energy analysis firm Montel Group, said: ‘This potential lack of resilience at a critical national and international infrastructure site is worrying.

‘An airport as large and as important as Heathrow should not be vulnerable to a single point of failure.’

Heathrow has its own biomass power plant and diesel backup generators, but they can power only essential safety systems, such as lighting and exit doors.

The airport’s electricity usually comes from three substations, each of which has a backup transformer. 

In the case of the damaged North Hyde substation, in west London, its backup transformer was also lost in the fire, which started shortly before midnight on Thursday.

Within hours, theories were swirling that sabotage, possibly even by Russia, may have been the cause. 

Despite the involvement of counter-terror officers, Scotland Yard said it was ‘not treating this incident as suspicious, although enquiries do remain ongoing’.

‘Various specialist investigators continue to examine the scene and it is expected to take some time before full assessments can be completed,’ a spokesman said.

The Politico website said those ‘familiar with the investigations’ were pointing towards a mistake by an electrical engineer sparking the fire. ‘It’s always cock-up rather than conspiracy,’ a source said.

London Fire Brigade last night said the fire ‘is believed to be non-suspicious’.

Offering a grovelling apology, Heathrow chief executive Thomas Woldbye said the airport had faced a situation ‘as bad as it gets for us’.

Supply had to be ‘restructured’ to draw on the other two substations, which involved restarting thousands of different systems – which takes ‘an enormous amount of time’, he said. 

‘We are very sorry about all the inconvenience. We lost a major part of our power supply. This was an incident of major severity. We lost power equal to that of a mid-sized city.

‘Our backup systems have been working as they should, but they are not sized to run the entire airport. This is unprecedented. Everyone has been shocked but all our procedures have been working as they should.’ 

Mr Woldbye insisted Heathrow cannot guard itself ‘100 per cent’ against such incidents. The airport would be up and running today and passengers should go the airport at the time they normally would, he added.

Mr Woldbye’s full pay has not been published since he took over the job in 2023, but his predecessor John Holland-Kaye earned £6.4 million in his final year.

Downing Street said Mr Woldbye would have ‘questions to answer’, as the closure of Heathrow costs Britain around £20 million a day. 

As well as catering for hundreds of thousands of passengers, the airport is also the gateway to 4,300 tons of cargo a day, worth £543 million.

A No 10 spokesman said: ‘There’s absolutely no doubt that there are questions to answer on how this has happened and what can be done to prevent the scale of disruption we’ve seen from happening again.’ 

BA chief executive Sean Doyle warned that its services will be severely impacted over the coming days, with more than 100,000 customers on 670 planes unable to fly yesterday.

The fire came a day after energy regulator Ofgem announced a £4 billion investment in plans to boost Britain’s under-pressure power network.

A 2022 report for the Mayor of London showed that the North Hyde substation was, at times, running at 106.2 per cent its rated capacity.

 A substation is responsible for stepping-down high-voltage electricity from suppliers and distributing it to homes and businesses.

Dr Conor Murphy, of grid analytics firm NovoGrid, said overloaded transformers were a common cause of fires at substations. 

‘The oil-cooled equipment in substations like this poses inherent fire risks, particularly from ageing infrastructure or overloaded system,’ he added. 

‘Complete airport shutdowns due to power failure are extremely rare. The incident underscores broader grid vulnerabilities. 

‘The cause of the fire will need to be determined to prevent similar events in the future – whether it was an electrical fault, human error or external factors.’

He added that it may be a year before the substation was restored because highly specialised equipment will need replacing.

Energy Secretary Ed Miliband said the complete shutdown of Heathrow made the airport look ‘quite vulnerable’.

Transport Secretary Heidi Alexander defended the airport while acknowledging the ‘immense distress and disruption’ its closure caused to passengers.

Justine Bayley, chairman of Stop Heathrow Expansion, said: ‘Just how competent are Heathrow’s bosses? We should not put all our eggs in that basket.

‘If we want to increase UK connectivity to the rest of the world, today’s fiasco has shown we need to spread the load and the risk to other airports.’