Heathrow was in full operation on Saturday, having been shut over a loss of power for 16 hours on Friday - but some passengers have continued to face disruption.
On Saturday morning, a Heathrow spokesperson said the airport was "open and fully operational" but warned passengers to still check with their airline before travelling.
It came after a fire at a nearby electrical substation late on Thursday night triggered a "significant power outage" on Friday, leading to more than 1,000 flights being cancelled and disrupting the travel plans of as many as 290,000 passengers at Europe's busiest airport.
Follow updates on Heathrow shutdown aftermath
Aftershocks were still felt by some passengers on Saturday, with flight boards regularly showing some cancellations and delays.
That was despite extra staff being drafted in and more flights added "to facilitate an extra 10,000 passengers".
Police have said the cause of the fire is believed to be non-suspicious, while London Fire Brigade (LFB) said its investigation was focusing on electrical distribution equipment.
The government has ordered an "urgent" investigation into what happened. Energy Secretary Ed Miliband instructed the National Energy System Operator (NESO) to look into the incident "so that it's prevented from ever happening again".
Heathrow boss Thomas Woldbye said the airport welcomed the independent investigation and would support "every effort to understand the causes and impacts" of the incident.
"We are committed to working closely with all stakeholders to ensure a thorough investigation to help strengthen the airport's future resilience," he said in a statement on Saturday afternoon.
The airport also announced it will launch an internal review led by Ruth Kelly, former transport secretary and an independent member of Heathrow's board.
Its chairman Lord Paul Deighton said in a statement: "Closing the airport yesterday had significant impacts for our passengers, our customers, our colleagues and the country.
"Heathrow regrets the disruption this caused. We hope that all those affected understand that the decision was made in order to prioritise the safety of our passengers and colleagues.
"We are committed to finding any potential learnings from this unprecedented incident."
Passengers still facing disruption
Passengers continued to share their frustrations with Sky News on Saturday over their experiences with delays and cancellations, which were expected to go on until at least the end of the weekend.
One was in a taxi bound for Heathrow when she found out the airport was closed.
Ray had been going to see family in Algeria to celebrate Ramadan and Eid and had travelled down from Manchester via train at 7am and hadn't checked the news - so arrived in London unaware of any issues.
"I noticed the underground was disrupted, so I took a taxi there," she told Anna Jones on Sky News Breakfast.
"As I was in the taxi, I found out the news that Heathrow was closed, so I had to stop the taxi driver in the middle of the trip. I wasn't sure what to do."
She called her dad, who was in the process of driving for four hours to Algiers airport to pick her up.
When she called the airline, "they were not very optimistic about being able to reschedule" as all the next flights were full, and all the direct flights to Algeria from the UK went via Heathrow.
She said she was looking for an alternative way to get to Algeria via mainland Europe, but added the flights were "very, very expensive".
Another passenger, John Climpson, was meant to fly out to the north of Sweden from Heathrow on Friday morning before competing in a 240km husky sledding challenge in the Arctic Circle.
But his flight was cancelled, meaning he had to wait to catch a flight from Heathrow to Stockholm on Saturday night, staying there overnight and then flying to Kiruna, the furthest north city in Sweden, on Sunday morning.
He said he and his team would arrive two days later than planned, meaning "all of our training time has disappeared and we are going to have to go straight into [the] 240km husky sledding challenge immediately, otherwise we won't make it in time".
He added Heathrow's Terminal 2 was "probably the quietest" he had ever seen it.
In a statement, transport secretary Heidi Alexander warned those planning to fly over the next few days that "some disruption" is expected to continue as "things get back to normal".
She urged anyone travelling to check with their airlines and plan their journeys.
British Airways says 90% of its flights went ahead
British Airways - Heathrow's biggest airline - said 90% of its scheduled flights went ahead on Saturday, which was above its expectations of about 85%.
In a statement on X, it said "the vast majority of our customers were able to get to where they needed to be despite the very challenging circumstances going into the start of the day".
"As things stand, we're anticipating being able to run a near-full schedule on Sunday and would therefore ask customers to come to the airport as normal unless we get in touch to tell them otherwise," it said.
Read more:
Heathrow shutdown is embarrassing at best
How Heathrow closure ruined travel plans
'Nightmare' Heathrow shutdown in numbers
How much could Heathrow closure cost UK economy?
The airline added it was "really grateful to our customers for bearing with us over the past couple of days, even though many of them have had their travel plans unacceptably cancelled or disrupted" and thanked its employees for working "so hard to support customers and help get our operation back on track".
Virgin Atlantic, another prominent airline at Heathrow, said on Saturday evening its operation had "fully stabilised" after a "significant impact" to its schedule over the past two days.
"We are still working to ensure all those impacted can complete their journeys as quickly as possible and we expect to run our full flying schedule on Sunday 23 March," it added.
"We're incredibly sorry for any disruption to our customers' journeys," the airline said in an earlier statement.
Flight delayed or cancelled? What are your rights?
Heathrow CEO Thomas Woldbye also apologised to passengers whose journeys had been disrupted, describing the delays "as big as it gets for our airport" and admitting "we cannot guard ourselves 100%".
However, he stressed the airport would not have shut unless there were "severe safety concerns".
He said a back-up transformer had failed, meaning systems had to be closed in line with safety procedures so power could be restructured from two remaining substations.
A National Grid spokesperson said teams had been working tirelessly to get the situation back under control: "Power supplies have been restored to all customers, including Heathrow, allowing operations to resume at Heathrow.
"We are deeply sorry for the disruption caused and are continuing to work closely with the government, Heathrow and the police to understand the cause of the incident."
? Listen to Sky News Daily on your podcast app ?
The disruption began late on Thursday night when fire crews were called to a blaze in Hayes, west London, at the electricity substation serving Heathrow and local properties.
LFB deputy commissioner Jonathan Smith told reporters that the fire involved a transformer - a key part of the substation - with 25,000 litres of cooling oil "fully alight".
Pictures from the scene showed large flames and plumes of thick black smoke, with around 70 firefighters and 10 engines working to extinguish the blaze.
Crews evacuated 29 people from neighbouring properties but there were no casualties.
In all, 67,000 households were left without power after the fire at the substation, but all supplies were restored by 2pm on Friday.
(c) Sky News 2025: Heathrow resumes flights amid internal review, 'urgent' govt investigation and further delays