Tens of thousands of British Airways frequent flyer accounts have been accessed by hackers.
The airline froze affected accounts to resolve the issue. BA said no personal information was viewed or stolen.
It said only a small proportion of its millions of customers were affected, and that their names, addresses, bank details and other personal information had not been accessed.
It is not known who is behind the hack, but it is believed to have been carried out by an automated computer program looking for ways through the carrier’s online security systems, according to a Press Association report.
A spokesman for the airline said: “British Airways has become aware of some unauthorised activity in relation to a small number of frequent flyer executive club accounts.
“This appears to have been the result of a third party using information obtained elsewhere on the internet, via an automated process, to try to gain access to some accounts.
“We would like to reassure customers that, at this stage we are not aware of any access to any subsequent information pages within accounts, including travel histories or payment card details.
“We are sorry for the concern and inconvenience this matter has caused and would like to reassure customers that we are taking this incident seriously and have taken a number of steps to lock down accounts so they can no longer be accessed.”