British Airways says it has fixed the IT glitch which caused more than 100 flights to be cancelled and more than 200 others to be delayed.
The airline said flights were returning to normal but warned there may be knock-on operational disruption.
At least 117 flights had been cancelled at Heathrow Airport, with 10 cancelled at Gatwick Airport.
The problem had caused BA to revert to using manual systems for check-in at airports, causing long queues.
It has apologised for the disruption and said customers on short-haul services from Heathrow, Gatwick and London City can rebook another day.
The airline said the issue had not been a global problem, and involved two separate systems - one which deals with online check in, the other that deals with flight departures.
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