Holidaymakers travelling with British Airways are facing delays and problems checking in online after an IT glitch.
At least 81 flights have been cancelled at Heathrow Airport, with 10 cancelled at Gatwick Airport. More than 200 other flights have been delayed.
BA said it is reverting to manual systems for check-in at airports.
It has apologised for the disruption and said customers on short-haul services from Heathrow, Gatwick and London City can re-book another day.
“We are working as quickly as possible to resolve a systems issue which has resulted in some short-haul cancellations and delays from London airports,” the airline said in a statement.
“A number of flights continue to operate,” it added, apologising to passengers impacted by the disruption and noting back-up manual systems were being used to keep many services in place.
Frustrated passengers took to social media to vent their anger and demand answers from BA, while photos shared showed airport terminals filled with stranded travellers.