The closure of London’s Heathrow airport due to a power outage is tipping one of the world’s busiest airports and the travel plans of hundreds of thousands of people into chaos. The airport will be shut all Friday after a blaze at an electrical substation near Heathrow.

Disruptions would continue “over the coming days and passengers should not travel to the airport under any circumstances,” Heathrow said. The airport said it can’t say when power will be reliable restored. The fire service said the cause of the blaze wasn’t yet known.