Britain's Heathrow airport -- Europe's busiest -- shut down early on Friday after a major fire at an electricity substation cut power to the sprawling hub, causing disruption worldwide to hundreds of flights and thousands of travellers.

Around 120 Heathrow-bound planes were in the air when the closure was announced, according to online flight tracking service FlightRadar24.

Around 70 firefighters were deployed to tackle the "highly visible" blaze at the substation in Hayes, west London.

Videos on social media, apparently shot inside Heathrow's terminals, showed shuttered shops and deserted corridors, lit only by emergency lighting.

Heathrow, which handles more than 80 million passengers a year, has around 1,300 takeoffs or landings a day.

British Airways said the closure of its main hub "will clearly have a significant impact on our operation and our customers".

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

The UK's second busiest airport, Gatwick, said it would accept some flights from Heathrow.

Others were diverted to Shannon in southwestern Ireland.

"We are aware of the........

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