London’s Heathrow Airport will remain closed on Friday after a fire at an electrical substation triggered a major power outage, disrupting operations at one of the world’s busiest airports and throwing the travel plans of hundreds of thousands into disarray.

“The airport is experiencing a significant power outage due to a fire at an electrical substation that supplies Heathrow,” the airport operator said in a statement posted on its website shortly before 3 am local time.

Passengers have been urged not to travel to the airport and to contact their airlines for updates.

Heathrow, the primary hub for carriers like British Airways and Virgin Atlantic, typically handles over 1,400 flights and around 200,000 passengers each day. Some inbound flights have already been diverted, including a Qantas Airways flight from Perth, which is now being rerouted to Paris, according to tracking service FlightRadar24.