Some flights were cancelled or delayed as the travel industry scrambled to reroute passengers and fix battered airline schedules after the huge fire at an electrical substation serving the airport.
Resumed flights had begun on Friday evening, but the shuttering of the world's fifth-busiest airport for most of the day left tens of thousands searching for scarce hotel rooms and replacement seats while airlines tried to return jets and crew to bases.
Operations were normal on Saturday morning but airlines were still dealing with the aftermath, airport chief executive Thomas Woldbye said.
"We don't expect any major amount of flights to be cancelled or delayed. There are some cancellations and there are some delays. We are handling them in the same way as we would normally do," Woldbye told BBC radio.
The vast majority of scheduled morning flights departed successfully on Saturday morning, with a handful of delays and cancellations, Heathrow's departures website showed.
British Airways, whose main hub is Heathrow, said it expected about 85 per cent of its schedule of almost 600 departures and arrivals to proceed on Saturday.
"We are planning to operate as many flights as possible to and from Heathrow on Saturday, but to recover an operation of our size after such a significant incident is extremely complex," the airline said in a statement.
"It is likely that all travelling customers will experience delays as we continue to navigate the challenges posed by Friday's power outage at the airport."
A Heathrow spokesperson said the airport had "hundreds of additional colleagues on hand in our terminals and we have added flights to today's schedule to facilitate an extra 10,000 passengers travelling through the airport".
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.
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.
Woldbye, asked on Friday who would pay for the disruption, said there were "procedures in place", adding "we don't have liabilities in place for incidents like this".
Restrictions on overnight flights were temporarily lifted by Britain's Department of Transport to ease congestion, but British Airways chief executive Sean Doyle said the closure was set to have a "huge impact on all of our customers flying with us over the coming days".
Airlines including JetBlue, American Airlines, Air Canada, Air India, Delta Air Lines, Qantas, United Airlines, British Airways and Virgin were diverted or returned to their origin airports following the closure, according to data from flight analytics firm Cirium.
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.
Prices at hotels around Heathrow jumped, with booking sites offering rooms for Stg500 ($A1000), roughly five times the normal price levels.
Police said after an initial assessment, they were not treating the incident at the power substation as suspicious, although inquiries remained ongoing.
London Fire Brigade said its investigations would focus on the electrical distribution equipment.
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.
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