More than a third of British Airways flights from Heathrow have been cancelled as disruption to passengers enters a second day.
The airline was hit by a worldwide computer system power failure on Saturday, causing cancellations and delays for thousands of passengers.
Sunday departures from Gatwick are delayed, but none have been cancelled.
The airline urged people to check the status of flights before before travelling to the airport.
Heathrow Airport is providing an updated schedule on its website, as is Gatwick.
BA apologised to customers for the issue, which is thought to have been caused by a problem with the IT system’s power supply.
Between 06:00 and 11:00 BST on Sunday, 143 British Airways flights were scheduled to depart from London Heathrow and Gatwick Airport.
So far 90 flights have left Heathrow and 36 have been cancelled.
At Gatwick, 17 planes have departed and no flights have been cancelled.
The airline is liable to reimburse thousands of passengers for refreshments and hotel costs.
Customers displaced by flight cancellations can claim up to £200 a day for a room (based on two people sharing), £50 for transport between the hotel and airport, and £25 a day per adult for meals and refreshments.
There have been reports that some passengers who departed from Heathrow on Saturday found their luggage was not at their destination when they landed.
Terry Page, 28, arrived in Fort Worth, Texas after delays and claimed that “about 50” passengers did not have their check-in luggage.
Thousands of bags remain at Heathrow Airport, but BA has advised passengers not to return to collect them, saying they will be couriered to customers.
The airport also said passengers whose flights were cancelled should not travel to Heathrow unless they have already rebooked their flight.
A spokesman said staff were “continuing to give passengers free water and snacks”.
On Saturday passengers described “chaotic” scenes at the airports, with some criticising BA for a lack of information.
The airline apologised and said it was refunding and rebooking customers.
The company’s chief executive Alex Cruz had said it was believed “the root cause [of the computer problems] was a power supply issue”. The company said there was no evidence the failure was the result of a cyber attack.
Other airlines flying in and out of the two airports were unaffected.
The IT failure affected check-in and operational systems, including customer service phone lines.
BA said although some of its IT systems had returned, “there will be some knock-on disruption to our schedules as aircraft and crews are out of position around the world.
“We are repositioning some aircraft during the night to enable us to operate as much of our schedule as possible throughout Sunday.”
A spokesman added: “We are extremely sorry for the huge disruption caused to customers throughout Saturday and understand how frustrating their experiences will have been.
“We are refunding or rebooking customers who suffered cancellations on to new services as quickly as possible and have also introduced more flexible rebooking policies for anyone due to travel on Sunday and Monday who no longer wishes to fly to/from Heathrow or Gatwick.”
The GMB union had suggested the failure could have been avoided, had the airline not outsourced its IT work.
BA denied the claim, saying: “We would never compromise the integrity and security of our IT systems”.
Aviation expert Julian Bray told the BBC the IT failure had an impact on planes taking off, as well as baggage systems, and staff access to computers.
“This is a very serious problem, they should have been able to switch to an alternative system – surely British Airways should be able to do this,” he said.
BA aircraft landing at Heathrow were unable to park as outbound aircraft could not vacate the gates, which resulted in passengers being stuck on aircraft.
Delays were also reported in Rome, Prague, Milan, Stockholm and Malaga due to the system failure, which coincided with a bank holiday weekend and the start of the half-term holiday for many people in the UK.
EU flight delay rights
- If your flight departed the European Union or was with a European airline, you might have rights under EU law to claim if the delay or cancellation was within the airline’s control
- Short-haul flights: 250 euros for delays of more than three hours
- Medium-haul flights: 400 euros for delays of more than three hours
- Long-haul flights: 300 euros for delays of between three and four hours; and 600 euros for delays of more than four hours
- If your flight’s delayed for two or more hours the airline must offer food and drink, access to phone calls and emails, and accommodation if you’re delayed overnight – including transfers between the airport and the hotel