London’s Heathrow Airport, one of the world’s busiest, has announced a complete shutdown all day today due to a “significant power outage” as a result of a large fire nearby.

The airport stated that “passengers should not travel to the airport under any circumstances until it reopens.”

It also warned that significant disruptions are expected over the coming days.

“To maintain the safety of our passengers and colleagues, Heathrow will be closed until 23h59 on 21 March,” Heathrow Airport said in a statement on X. That is 11am AEDT on Saturday.

Flights to and from Heathrow are now being diverted or cancelled.

Flight tracking website FlightRadar24 said 120 flights currently in the air “will be diverting to alternate airports or returning to their origins.”

The first scheduled arrival since the closure is a British Airways flight from Johannesburg.

It was due to land at 4.30am local time (3.30pm AEST) and is still listed as “expected” on the Heathrow website.

A United Airlines flight from New York has been diverted to Ireland, according to United’s website.

Flights to Zurich, Paris and Madrid have been cancelled, according to Heathrow’s listings.

Cause of fire unknown, authorities say 

The London Fire Brigade said in a statement that a transformer within an electrical substation in Hayes, in west London, was alight. Ten fire engines and around 70 firefighters were responding to the blaze, it added.

Around 150 people have been evacuated and authorities have set up a 200-meter cordon as a precaution, Assistant Commissioner Pat Goulbourne said in the statement. Firefighters have led 29 people from surrounding properties to safety.

Fire officials said they were called to the fire around 11.23pm local time on Thursday and crews have been on scene throughout the night.

The cause of the fire is not yet known, they said.

Videos shared on social media showed huge flames and smoke rising into the air early Friday.

“As we head into the morning, disruption is expected to increase, and we urge people to avoid the area wherever possible,” Goulbourne said.

Scottish and Southern Electricity Networks said they are aware of a “widespread power cut” in the London suburbs of Hayes, Hounslow and surrounding areas, near the airport. According to its website, the unplanned outage was affecting more than 16,000 homes.

Passengers are advised not to travel to Heathrow and should contact their airline for further information, the airport said in its statement.

Videos shared on social media showed Heathrow Airport largely dark amid the power outage.