British Airways has admitted its frequent flyer accounts have been hacked but insists no personal details have been stolen.
The airline says it has had to freeze the passenger accounts while the issue is resolved which means customers may not be able to use their air miles.
BA said only a small number of its millions of customers have been affected, and that their names, addresses, bank details and other personal information have not been accessed.
The company apologised to customers and said it hoped to have the system back up and running soon.
A British Airways 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."