British Airways' passengers have suffered more disruption as computer check-in problems continued to cause delays.
Average delays of 30 minutes on all services were expected as passengers were being checked in manually.
BA added that the technical problem is being dealt with and that delays are starting to shorten.
The airline advised customers to arrive for flights as normal, and said the computer system should be working again within the next couple of days.
Mike Street, BA's director of customer services and operations, said: "We would like to apologise to all our customers and reassure them that we are doing everything we can to minimise the inconvenience.
"We have laid on extra staff in the terminals and most flights are now getting away with minimal delays.
"The situation is under control and the delays are beginning to shorten. In the meantime, we are offering our customers the chance to change their travel plans if they wish."
The technical hitch affected all BA check-in areas worldwide and caused disruption to flights on Friday, with the cancellation of three services from Heathrow's Terminal 4.
On Thursday, passengers bound for Vancouver, Canada, had to be put up in hotels around Heathrow airport overnight and another plane to Kuala Lumpur in Malaysia was cancelled.
BA has set up a free telephone line for customers requiring additional information. The number is 0800 727 800.