See BTN Europe for the latest updates to this report.
UPDATE: The airport in a Friday statement said it was able to safely restart flights. "To maintain the safety of our passengers and colleagues, Heathrow will [be] running a reduced operation prioritising repatriation and relocation of aircraft," it added.
London's Heathrow Airport will be closed Friday, March 21, until midnight due to a power outage caused by a nearby fire, leading to the cancellation of hundreds of flights and disruption likely to continue over the weekend.
A statement posted by the airport on social media at around 4:30 a.m. local time said: "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.
"Passengers are advised not to travel to the airport and should contact their airline for further information. We apologise for the inconvenience."
All flights from Heathrow have been canceled today, while incoming flights are being cancelled or diverted, although London's other main airports – Gatwick, Stansted and Luton – have limited capacity to help. Some flights have been diverted to airports further afield across the U.K. and northern Europe.
More than 1,330 flights and up to 291,000 passengers are likely to be affected today, according to aviation analytics firm Cirium.
The airport, which is a major hub for transatlantic travel, was scheduled to see 665 departures on Friday, while a further 669 flights were due to arrive. British Airways has the largest share of flights from Heathrow—around 50 percent of all departures, according to Cirium—followed by Virgin Atlantic and Lufthansa.
In a statement on its website, British Airways said: "Customers due to travel from Heathrow on Friday are advised not to travel to the airport until further notice. We are closely monitoring the situation and will provide more information as soon as we have further details."
Writing on LinkedIn, Clive Wratten, CEO of the U.K.'s Business Travel Association, said: "It will be chaotic [but] try and be patient, and history suggests this industry of ours is incredibly effective at recovering from a major incident such as this."
The power outage has also affected thousands of local residents and transport links, including Heathrow Express train services which connect the airport with central London.