Heathrow airport was completely shut down on Friday after a power outage due to a fire at a London electricity substation, causing travel chaos for passengers around the world and the cancellation or diversion of hundreds of flights from Europe's busiest air hub.

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.

Firefighters were called to a "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 around 0800 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 shut-down left many passengers struggling to make alternative arrangements.

British Airways said the closure of its main hub would have a "significant impact" on its operations and customers.

"We're working as quickly as possible to update them on their travel options for the next 24 hours and beyond," it said.

Flights diverted

Around 120 Heathrow-bound planes were in the air when the closure was announced, according to Flightradar24.

Airport authorities said they "expect significant disruption" over the coming days.

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.

One resident near the substation said she heard a "massive explosion" as the power cut out just before midnight while a night shift worker reported seeing a "bright flash of white".

Around 150 people were evacuated from nearby properties because of the fire

London Fire Brigade deputy commissioner Jonathan Smith told reporters firefighters worked "tirelessly in challenging and very hazardous conditions" to bring the flames under control.

He said 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".

The outage left 100,000 homes without power overnight, Energy Secretary Ed Miliband said. By morning, power had been restored to all but around 4,000.

As the scale of the disruption began to emerge, the minister faced questions about how one fire could shut down an entire airport.

He said 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.

The chair of the parliamentary transport committee, Ruth Cadbury, said it was "speculative" to suggest at the moment that arson might have caused the fire.

But she told Times Radio: "There are obviously questions about it."

Disruption

Flights from all over the world were affected by the closure and aviation consultant Philip Butterworth-Hayes told AFP it would cost "certainly more than £50 million ($64.7 million)" to the airport and airlines.

In January, the UK government gave permission for Heathrow to build a third runway -- which could be ready by 2035 -- after years of legal disputes due to opposition from local residents.

Five major airports serve the British capital and towns nearby, but capacity is stretched, especially at Heathrow, whose two runways each measure almost four kilometres (2.5 miles) in length.

The airport covers a total area of 12.3 square kilometres (4.8 square miles).

Situated 25 kilometres west of central London, Heathrow's main flight destinations last year were Dublin, Los Angeles, Madrid and New York.

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AFP