A fire at an electrical substation near London’s Heathrow Airport has closed one of the world’s busiest airports.

Heathrow Airport said the fire at the substation, which supplies power to the airport, has caused a “significant power outage” and the airport is expected to be closed until 11:59 p.m. local time on Friday night.

“This is a highly visible and significant incident, and our firefighters are working tirelessly in challenging conditions to bring the fire under control as swiftly as possible,” Pat Goulbourne, assistant commissioner of the London Fire Brigade said in a statement shortly before 2 a.m. local time.

The fire has caused a power outage affecting a large numbers of homes and businesses and led to some evacuations, the fire brigade said.

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

This is a developing story. Please check back for updates.