London's Heathrow Airport on Friday advised passengers not to attempt to travel to the aviation hub "under any circumstances" after a fire at an electricity substation cut power to the airport.
The blaze grounded all flights and shut down the airport, causing misery for hundreds of thousands of passengers.
Flight tracking service FlightRadar24 said at least 1,351 flights to and from the United Kingdom's main airport would be affected.
In terms of the most canceled flights, the worst affected airlines from Friday's disruption are British Airways, Virgin Atlantic, Lufthansa, American Airlines and Air Lingus.
All train services to and from the airport were canceled.
The airport operator warned the facility would be closed for the entire day and that it expected "significant disruption over the coming days."
How are passengers being impacted by the closure?
Several tracking services showed some Heathrow-bound flights were diverted to London's second airport, Gatwick
Other flights already in the air were diverted to other European airports, including Paris, Frankfurt, Amsterdam and Dublin.
One passenger, Adrian Spender, posted on X from midair on an Emirates Airline Airbus A380, saying: "#Heathrow no idea where we are going yet. Currently over Austria." His plane later landed at Manchester Airport, he wrote.
Another flier, Lawrence Hayes, was three-quarters of the way to London from New York when his Virgin Atlantic flight was diverted to Glasgow.
"It was a red-eye flight and I'd already had a full day, so I don't even know how long I've been up for," Hayes told the BBC, adding that he would now be traveling to London by train, which typically takes five hours.
UK media reported that some airlines who had rerouted flights elsewhere in Europe were arranging buses to allow passengers to continue to London.
Budget carriers Ryanair and Easyjet said they were offering additional capacity in and out of other UK airports over the weekend to offer alternatives for passengers affected by Heathrow's sudden closure.
Several flights from long-haul destinations were canceled before take-off or landed at stop-off points en route to await diversion instructions. Others returned to their airports of origin.
Ed Miliband, Secretary of State for Energy and Climate Change, told Sky News that Heathrow's closure would cause "terrible disruption" for thousands of passengers.
Passengers, meanwhile, reported delays in reaching some airlines by phone to inquire about refunds or rebookings.
Aviation experts have warned of further disruptions to flights over the weekend due to the knock-on effects of the closure.
"It's going to take five days to fix this because the crews and planes are now in the wrong place," Andrew Charlton, managing director of the Swiss-based aviation consultancy Aviation Advocacy, told DW.
What could be the economic impact of the shutdown?
The closure is expected to result in losses totaling tens or hundreds of millions of pounds for Heathrow and the affected airlines.
For comparison, a technical malfunction in the UK's air traffic control system led to widespread flight cancellations in August 2023, which cost airlines around $127 million (€117.2 million).
During short-term strike action, which caused similar closures at German airports last year, Lufthansa said it had suffered losses of €250 million in just three months, Reuters news agency reported.
Aviation consultant John Strickland told the UK's Press Association news agency that the Heathrow shutdown would have a "massive impact in lost revenues and disruption costs, primarily for the airlines."
Strickland said it was "unlikely" that airlines would be able to recoup their losses from refunds, re-bookings and accommodating stranded passengers in hotels.
Aviation Advocacy's Charlton, meanwhile, noted that as well as the extra costs passengers may face, the disruption would negatively impact cargo carried in the bellies of aircraft.
"[Aviation cargo] is often just-in-time, high-value, perishable goods," Charlton told DW. "So sending it to the wrong location is a major problem."
How important is Heathrow to the UK economy?
Situated on the western side of the UK capital, Heathrow is the world's fourth-busiest airport after Atlanta, Dubai and Tokyo, serving more than 230 destinations.
Typically a plane departs or lands at Heathrow every 45 seconds.
Earlier this year, the airport had its busiest January on record, handling more than 6.3 million passengers, 5% more than the same month last year.
Heathrow has been operating at 99% capacity, with almost 84 million passengers using the airport in 2024.
That compares with about 70 million accessing Paris' Charles de Gaulle airport, nearly 67 million at Amsterdam's Schiphol and around 66 million entering Madrid's main aviation hub.
Heathrow recently won government backing to open a third runway and will submit its formal proposal for the project this summer, which is hoped will boost trade and economic growth.
The airport risks being overtaken by European rivals. For example, Paris' Charles de Gaulle and Frankfurt Airport each have four runways, with six at Amsterdam's Schiphol.
Heathrow employs more than 76,000 people. More than a hundred thousand others have jobs with suppliers, contractors and other firms operating around the airport.
