Flights will resume at Heathrow Airport after a "catastrophic" fire cut power, causing travel chaos for hundreds of thousands of people.
Counter-terrorism police are leading an investigation into the blaze, which broke out at a nearby electrical substation overnight on Friday, local time.
The London Fire Brigade later told Reuters the fire is believed by police to be non-suspicious.
Some 1,300 flights and hundreds of thousands of travellers worldwide were affected by the closure of Europe's busiest airport.
The blaze also caused thousands of homes in the area to lose power.
A Heathrow Airport spokesperson apologised for the inconvenience caused by the fire and said its priority was to repatriate passengers whose flights were diverted.
"Our first flights will be repatriation flights and relocating aircraft. Please do not travel to the airport unless your airline has advised you to do so," the spokesperson said.
"We will now work with the airlines on repatriating the passengers who were diverted to other airports in Europe. We hope to run a full operation tomorrow [Saturday local time] and will provide further information shortly."
"As the busiest airport in Europe, Heathrow uses as much energy as a small city, therefore getting back to a full and safe operation takes time. We apologise for the inconvenience caused by this incident."
The Metropolitan Police said there was no evidence of "foul play" but it was keeping an "open mind" as to the cause of the fire. Scotland Yard's counter-terrorism unit was drafted in to lead the investigation.
Qantas flights to and from Heathrow were affected by the closure, with some passengers diverted to Paris and then sent to London by bus.
[TWEET]Others were left grappling with ruined plans, with one man telling the ABC his honeymoon to the Caribbean was derailed by cancelled flights.
"It was the trip of a lifetime really," he told the ABC outside the airport.
"It's just a nightmare, it's ruined the whole experience. Not happy, not happy at all.
"We're supposed to be in out first-class seats now, but obviously we're not, and now we've gotta book what we can and get where we can. It's shattering."
Another couple told the ABC they were off to Saint Lucia.
"We're just going back home now because they don't fly every day. I don't think they'll get us on the plane every couple of days so we may have lost the whole holiday," one woman said.
Heathrow Airport's planning under scrutiny
Ed Miliband, the UK's energy secretary, called the incident "catastrophic" and "unprecedented".
"I’ve spoken to the National Grid about what’s happened, which is a catastrophic fire at a sub-station that serves Heathrow," he said on Friday morning, local time.
Mr Miliband said the fire had disabled back-up power and that engineers had been working to deploy a third source.
The incident has raised questions about contingency planning at the world's fifth-biggest airport, however.
The shutdown comes less than a year after Heathrow told Britain's Civil Aviation Authority (CAA) in a filing that it was "a leader in airfield resilience".
The head of the global airlines body, The International Air Transport Association, Willie Walsh, was scathing.
"It is a clear planning failure by the airport," Mr Walsh said.
Heathrow has an operational resilience plan, aimed at identifying risks that could upset operations.
Heathrow chief executive Thomas Woldbye admitted that certain contingencies are often not enough to safeguard the airport all the time.
"This [power supply] is a bit of a weak point," Mr Woldbye told reporters outside the airport.
"But of course contingencies of certain sizes we cannot guard ourselves against 100 per cent, and this is one of them."
In 2023, Heathrow completed a new energy strategy, pledging more renewable energy "whilst protecting the resilience of our energy network," according to its latest annual report.
