Prepare for flight disruptions worldwide as Heathrow Airport has been closed following a fire, leaving thousands of passengers stranded.

Around 11 p.m. local time on March 20th, a fire broke out at a nearby electricity substation that serves Heathrow, causing a power outage that has halted flights in and out of one of the world’s busiest airports and affected Heathrow’s backup power supply. Investigators from London’s Metropolitan police counter-terrorism unit are looking into the cause of the fire to rule out any sort of foul play, though so far indications are that the fire was most likely caused by a circuit malfunction.

“Given the location of the substation and the impact this incident has had on critical national infrastructure, the Met’s Counter Terrorism Command is now leading inquiries,” the Metropolitan Police said in a public statement. “All possible lines of enquiry are being pursued and further updates will be provided in due course.”

In 2023, Heathrow was ranked as the world’s fourth busiest airport, and the airport closure is likely to cancel at least 1,300 flights on Friday and residual delays could affect at least 145,000 passengers in the upcoming days. 

The airport released a statement on X. “Due to a fire at an electrical substation supplying the airport, Heathrow is experiencing a significant power outage. To maintain the safety of our passengers and colleagues, Heathrow will be closed until 23h59 on 21 March,” the statement says. “Passengers are advised not to travel to the airport and should contact their airline for further information.”

Delta Air Lines, which canceled all 10 of its flights to Heathrow on Friday, has issued a travel waiver through Sunday for passengers who need to rebook their flights.