British Airways, whose main hub is Heathrow, said it expected around 85% of its schedule to proceed on Saturday, and chief executive Sean Doyle warned of a "huge impact on all of our customers flying with us over the coming days".
The airport, the world's fifth-busiest, had been due to handle 1,351 flights on Friday, flying up to 291,000 passengers. But the fire at a nearby electrical substation forced planes to be diverted to other airports and many long-haul flights returned to their point of departure.
Britain's energy ministry said on Saturday it had commissioned the National Energy System Operator to carry out an urgent investigation into the outage that raised questions about the resilience of the country's critical infrastructure.
Aviation experts said the last time European airports experienced disruption on such a large scale was the 2010 Icelandic volcanic ash cloud that grounded some 100,000 flights.
"It has been absolutely insane", said Amber Roden, a US citizen getting married in three days' time, after a number of her relatives had their flights cancelled.
Two relatives who were halfway to London from Atlanta had to turn around and go back, she said. Two others will not make it to the UK until the day of the wedding, which she has been planning for two years.
The vast majority of scheduled morning and early afternoon flights departed successfully on Saturday, with a handful of delays and cancellations, Heathrow's departures website showed.
"We don't expect any major amount of flights to be cancelled or delayed," Heathrow chief executive Thomas Woldbye told BBC radio.
The airport has hundreds of additional staff on hand to facilitate an extra 10,000 passengers travelling through the airport, a spokesperson said in a statement.
But airlines were still left dealing with disrupted schedules and the tens of thousands of passengers whose journeys had been interrupted.
Virgin Atlantic said on Saturday that it was planning to run a near-full schedule with limited cancellations. Air India said it had restarted flights to and from Heathrow and expected to operate "as per schedule".
FIRE NOT SUSPICIOUS
Several passengers travelling to Heathrow from London's Paddington Station were still nervous.
"I'm just hoping that when I get there, I can actually go." said university professor Melissa Graboyes, who said she was repeatedly checking the status of her flight to Toronto.
Police said that after an initial assessment they were not treating the incident as suspicious, although inquiries remained ongoing. London Fire Brigade said its investigations would focus on the electrical distribution equipment.
The travel industry, facing the prospect of a financial hit costing tens of millions of pounds and a likely fight over who should pay, questioned how such crucial infrastructure could fail without backup.
"It is a clear planning failure by the airport," said Willie Walsh, head of global airlines body IATA, who, as former head of British Airways, has for years been a fierce critic of the crowded hub.
Heathrow and London's other major airports have been hit by other outages in recent years, most recently by an automated gate failure and an air traffic system meltdown, both in 2023.
