Hackers have accessed tens of thousands of British Airways frequent-flyer accounts.

The airline said no personal information had been viewed or stolen and it had frozen affected accounts while it resolves the issue. It means top executive club flyers may not be able to use their points until the issue is resolved.

The airline said only a small proportion of its millions of customers were affected, and no names, addresses, bank details or other personal information had been accessed. BA apologised to customers and said it expected to have the system back up and running in the next day or so.

It is not known who was behind the hack, but it is believed to have been carried out by an automated computer program looking for vulnerabilities in the company’s online security systems.

A BA spokesman 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.”