In an earlier update on the disruption, the company said no flights had been cancelled.
Downdetector, a website that logs complaints with websites that are not working, indicated a sharp increase in problems with the BA site after 17:00.
It is not clear how widespread the disruption was, but it was reported to have affected flights globally, as well as some domestic UK flights.
Travel expert and journalist Simon Calder estimated the problems could have affected tens of thousands of passengers, although the delays seemed to be limited to one or two hours for affected flights.
One passenger wrote on X that they were “being made to stand on the tarmac in a very cold Verona waiting for an already delayed flight”.
Travellers in Hamburg, Naples, Lyon and Dusseldorf also reported delays.