Heathrow is supporting an independent probe announced by the government into its enforced shutdown on Friday after a fire at an off-site electricity substation.
The airport faces pressure from airlines to explain why flights were suspended for 18 hours after the fire in the early hours of Friday morning.
Arguments flared over the weekend over the decision to close the airport altogether amid doubts over its resilience as a global aviation hub.
The government ordered a six-week investigation into the closure, led by the National Energy System Operator.
Heathrow also confirmed that an internal review will be conducted, led by former transport secretary and independent board member Ruth Kelly.
National Grid chief executive John Pettigrew also weighed in, telling the Financial Times that Heathrow had enough power to using two working substations to avoid shutting down during the fire.
More than 1,000 flights were cancelled affecting tens of thousands of passengers worldwide. Heathrow was able to restore full operations just over 24 hours after the substation fire first took hold. The airport added 50 slots into its schedule to enable a further 10,000 passengers to travel on Saturday.
Heathrow confirmed that it would operate a full schedule of more than 1,300 flights on Sunday after handling over 250,000 passengers on Saturday.
A spokesperson said: “Heathrow is operating a full schedule today, with over 1,300 flights planned.
“Over the weekend, we successfully managed over 2,500 flights, serving over 400,000 passengers.
“We apologise for the inconvenience caused by Friday’s closure, following a significant fire at an off-site power sub station. Our ongoing priority remains serving our passengers and getting them safely and quickly away on their journeys.”
British Airways was able to deliver around 90% of its Saturday flying schedule, enabling the “vast majority” of passengers were able to travel get “despite the very challenging circumstances going into the start of the day”.
BA anticipated being able to run a “near-full” schedule on Sunday.
A spokesperson for the Heathrow-based carrier said: “We’re 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.
“We’d also like to thank the thousands of colleagues who have worked so hard to support customers and help get our operation back on track.”
Aviation analytics firm Cirium noted that arriving and departing flights had returned to normal on Sunday, with services “generally on time”.
But the firm added: “This does not mean that passengers were not impacted; in all likelihood the airlines and the airport will need a few more days to accommodate all disrupted passengers.”
Virgin Atlantic said: “Our operation has fully stabilised following the closure of Heathrow airport on Friday, which caused a significant impact to our flying programme over the past two days.
“We are still working to ensure all impacted customers can complete their journeys as quickly as possible.
“We have been operating our full flight schedule since Sunday.”
The blaze started in a transformer within the electrical substation in Hayes north of Heathrow around midnight on Thursday.
Transport secretary Heidi Alexander said: “This incident caused significant disruption but Heathrow, National Grid, and our emergency services have worked swiftly to get people travelling again.
“Heathrow is a massive airport that uses the energy of a small city, so it’s imperative we identify how this power failure happened and learn from this to ensure a vital piece of national infrastructure remains strong.”
The London hub’s chairman Lord Paul Deighton admitted that the closure of the airport had “significant impacts for our passengers, our customers, our colleagues and the country”.
He added: “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.”
He noted that the review by Kelly “will analyse all of the relevant material concerning the robustness and execution of Heathrow’s crisis management plans, the airport’s response during the incident and how the airport recovered the operation with the objective of identifying any improvements that could be made to our future resilience”.
Separately, Heathrow chief executive Thomas Woldbye welcomed the probe into the National Grid electrical substation fire “and on energy resilience for critical national infrastructure more broadly”.
He said: “We will support every effort to understand the causes and impacts of [Friday’s] off-airport incident and we are committed to working closely with all stakeholders to ensure a thorough investigation to help strengthen the airport’s future resilience.”
A Heathrow spokesperson said: “We have welcomed the government’s announcement of an investigation into the cause and response to the off-airport power outage and have launched a review, to be chaired by former transport secretary Ruth Kelly, of Heathrow’s response. While these inquiries are ongoing, our focus remains on serving our passengers.”
A spokesperson of the airport added: “As the National Grid’s chief executive, John Pettigrew, noted, he has never seen a transformer failure like this in his 30 years in the industry.
“His view confirms that this was an unprecedented incident and that it would not have been possible for Heathrow to operate uninterrupted.
“Hundreds of critical systems across the airport were required to be safely powered down and then safely and systematically rebooted.
“Given Heathrow’s size and operational complexity, safely restarting operations after a disruption of this magnitude was a significant challenge.
“In line with our airline partners, our objective was to reopen as soon as safely and practically possible after the fire. The emergency services and hundreds of airport colleagues worked tirelessly throughout Friday to ensure the safe reopening of the airport.
“Their success meant that over the weekend, we were able to focus on operating a full schedule of over 2,500 flights and serving over 400,000 passengers.
“Lessons can and will be learned, which is why we fully support the independent investigation announced by the government.”