Heathrow said its teams worked tirelessly to reopen the world's fifth-busiest airport late on Friday local time after it was forced to close entirely after a huge fire engulfed a nearby substation on Thursday night, with travellers told to stay away.
The airport had been due to handle 1351 flights on Friday, flying up to 291,000 passengers, but planes were diverted to other airports in Britain and across Europe, while many long-haul flights returned to their point of departure.
Heathrow said there would be a limited number of flights on Friday, mostly focused on relocating aircraft and bringing planes into London.
"Tomorrow morning, we expect to be back in full operation, to 100 per cent operation as a normal day," said Heathrow chief executive Thomas Woldbye on Friday.Â
"What I'd like to do is to apologise to the many people who have had their travel affected ... we are very sorry about all the inconvenience."
Police said that after an initial assessment, they were not treating the incident as suspicious, although enquiries remained ongoing. London Fire Brigade said its investigations would focus on the electrical distribution equipment.
The closure not only caused misery for travellers but provoked anger from airlines, which questioned how such crucial infrastructure could fail.
The industry is now facing the prospect of a financial hit costing tens of millions of pounds, and a likely fight over who should pay.
"You would think they would have significant back-up power," one top executive from a European airline told Reuters.
Heathrow's Woldbye said back-up systems and procedures had worked as they should.
"This (power supply) is a bit of a weak point," he 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."
Asked who would pay, he said there were "procedures in place", adding "we don't have liabilities in place for incidents like this".
British transport minister Heidi Alexander said the incident had been out of Heathrow's control.
"They have stood up their resilience plans very swiftly and have been working in close collaboration with all the emergency responders and the airline operators," she told reporters.
Airlines including JetBlue, American Airlines, Air Canada, Air India, Delta Air Lines, Qantas, United Airlines, IAG-owned British Airways and Virgin were diverted or returned to their origin airports in the middle of the night, according to data from flight analytics firm Cirium.
Passengers stranded in London and facing the prospect of days of disruptions were scrambling to make alternate travel arrangements.
"It's pretty stressful," said Robyn Autry, 39, a professor, who had been due to fly home to New York. "I'm worried about how much is it going to cost me to fix this."
Prices at hotels around Heathrow jumped, with booking sites offering rooms for STG500 ($A1030), roughly five times the normal price levels.
Prime Minister Keir Starmer's spokesperson said there were questions to answer about how the incident occurred and there would be a thorough investigation.
