British Airways (BA) cancelled all short-haul flights from London's Heathrow airport on Saturday morning after an IT failure.
The airline has said the problem, which came to light on Friday and affected its website, app, and airport operations, was not caused by a cyber attack but was a hardware issue. Luggage piled up and some customers were stuck on planes after the major outage.
BA has warned that further disruption is expected during the day due to the hardware issue.
"We are extremely sorry that due to the continuing technical issues we are facing we have regrettably had to cancel all short-haul flights from Heathrow today until midday," the airline said in a statement.
"Our long-haul services at Heathrow and all flights at Gatwick and London City Airport are due to operate as planned, but customers may experience some delays. Our website ba.com is working and customers can check-in online and at the airport," the airline said.
The airline has been caught up in tit-for-tat bans by Russia and Britain that have stopped their respective national carriers from using each other's airspace.
It advised passengers to check on its website for the latest information before travelling to the airport as more disruption was expected.
BA, owned by IAG, was hit by a major computer system failure in 2017 that stranded 75,000 passengers over a holiday weekend, sparking a public relations disaster and pledges from the carrier that it would do better in future.
The company said customers would be offered a full refund for cancelled services or could rebook at a later date for free.
"We know we have let our customers down and we will do everything we can to make this up to them – but for now our focus is on getting as many customers and flights away as we can."