The Heathrow airport in London was shut down on Friday after a fire at an electrical substation that supplies power to the facility. The closure led to significant flight disruption.

The fire at a substation in the nearby Hayes town led to a “significant power outage” at the airport.

Heathrow’s operator said that the airport will be closed till 11.59 pm on Friday, citing safety concerns. “Passengers are advised not to travel to the airport and should contact their airline for further information,” the operator said.

Heathrow is among the busiest airports in the world, handling about six million passengers every month.

About 120 flights that were already on their way to Heathrow were diverted to the Gatwick airport in London, Schiphol in Amsterdam and Paris’ Charles de Gaulle airport, among others, aircraft tracking website FlightRadar24 said on Friday morning.

The closure of the airport for the entire day will affect at least 1,351 flights, according to FlightRadar24. This excludes flights that might be cancelled or delayed due to aircraft “being out of position”, the website added.

Air India said that all its flights to and from Heathrow on Friday have been cancelled.