Heathrow plies routes to around 80 countries and some 1,350 flights had been due to land or take off from the airport’s five terminals on Friday, according to the online flight tracking website Flightradar24.
Some 230,000 passengers a day and 83 million a year use Heathrow, making it one of the world’s busiest airports.
London’s Metropolitan Police said the force’s Counter Terrorism Command was leading enquiries given the “impact this incident has had on critical national infrastructure.”
“While there is currently no indication of foul play we retain an open mind at this time,” a spokesman added.
Firefighters were called to the “highly visible” blaze at the substation shortly after 2320 GMT on Thursday and around 70 were sent to battle the fire, which was brought under control by 0800 GMT.
Witnesses saw water cannons were still being used at the smoking fire site at around 1300 GMT.
UK Energy Secretary Ed Miliband promised that the government would do all it could to restore power to Heathrow quickly, as questions were raised about the airport’s “resilience.”
The shutdown left many frustrated passengers struggling to make alternative arrangements.
Around 120 Heathrow-bound planes were in the air when the closure was announced, according to Flightradar24.
The UK’s second busiest airport, Gatwick, said it would accept some flights from Heathrow. Others were diverted to European airports including Shannon in southwestern Ireland, Frankfurt, and Paris Charles de Gaulle.
London Fire Brigade deputy commissioner Jonathan Smith said firefighters had worked “tirelessly in challenging and very hazardous conditions.”
He told reporters the fire had involved a “transformer comprising of 25,000 litres of cooling oil fully alight” posing a hazard due to the substation’s “still live high voltage equipment.”
One resident near the substation recalled hearing a “massive explosion” as the power cut out just before midnight while another reported seeing a “bright flash of white.”
Around 150 people were evacuated from nearby properties because of the fire.
The outage left 100,000 homes without power overnight. By morning, power had been restored to all but around 4,000.
As the scale of the disruption began to emerge, Miliband faced questions about how one fire could shut down an entire airport.
He said the electricity distribution network National Grid had told him they had not seen “anything like the scale” of what happened.
“But it makes Heathrow look quite vulnerable and therefore we’ve got to learn lessons, as I say, about not just Heathrow but how we protect our major infrastructure,” he told ITV News.
Willie Walsh, director general of the airline industry trade association IATA, said the closure was “yet another case of Heathrow letting down both travellers and airlines.”
“How is it that critical infrastructure – of national and global importance – is totally dependent on a single power source without an alternative? If that is the case – as it seems – then it is a clear planning failure by the airport,” he said on X.
Flights from all over the world were affected by the closure and aviation consultant Philip Butterworth-Hayes said it would cost “more than £50 million (US$64.7 million)” to the airport and airlines.
Situated 25 kilometres west of central London, Heathrow’s main flight destinations last year were Dublin, Los Angeles, Madrid and New York. (AFP)
