An IT glitch that caused the cancellation of more than 100 British Airways flights has been fixed, the airline says.
Tens of thousands of passengers trying to fly to or from Heathrow, Gatwick or London City airports were affected by the problem on Wednesday.
Some 117 flights due to depart or arrive at Heathrow were axed.
BA could face a compensation bill in excess of £8 million ($A14 million) if all those affected claim what they are entitled to under EU rules.
“We have resolved the temporary systems issue from this morning which affected a number of our flights today,” the airline said in a statement.
“We apologise to all our customers caught up in the disruption, and appreciate how frustrating their experience has been.
“Our teams have been working tirelessly to get the vast majority of customers on their way, with most of our flights departing.”
The statement added that flights are “returning to normal” but warned there may be some “knock-on operational disruption”.
There were long queues of passengers at Heathrow and error messages on the BA app as some services lagged more than five hours behind schedule.
The airline was forced to use back-up and manual systems in a bid to cope with the problem.
Darren Rowe of the Cotswolds said his flight to Hamburg from Heathrow for business meetings was cancelled before “all chaos let loose”.
“There were massive queues, it was queue here, queue there, nobody was saying anything. The lack of information was just pathetic,” he said.
“You’ve got young families in that queue, people going to weddings, birthdays, on business. They could have had somebody come around with water updating people about what was going on.”
BA has recently faced threatened strikes by pilots and is also set to be fined £183 million ($A330 million) over a cyber attack on its security systems last year in which the personal data of up to 500,000 customers was stolen.