British Airways has apologised after cancelling all short-haul flights from Heathrow Airport on Saturday morning.
The airline said ongoing technical issues were to blame.
Long-haul flights continued to operate but customers were warned there could be delays. Passengers travelling from Gatwick or London City Airport could also be affected.
British Airways warned it “anticipated further disruption during the day” due to the hardware issue.
The issues were not caused by a cyber attack, the airline said.
Most flights from Heathrow on Saturday afternoon are set to run as expected, but it is not yet known when the problems will be completely resolved.
Some flights are still being cancelled because of systems issues, and the not all planes and crew are in the right places because of the earlier issues with flights.
Anyone due to travel later is advised to check their flight status before travelling to the airport.
British Airways said it knew it had “let customers down” and would do everything it could “to make this up to them”.
“But for now our focus is on getting as many customers and flights away as we can,” the company added.
It is giving customers on cancelled services the option of getting a full refund. Passengers due to fly on short-haul services from Heathrow on Saturday can also choose to rebook for a later date, for free. The airline said it would be contacting customers “proactively”. (BBC)