British Airways said on Sunday tens of thousands of its frequent flyer accounts had been hacked and that it had frozen those affected to sort the problem out.
The airline said no personal information had been viewed or stolen, but warned customers that some fliers might not be able to temporarily use their air miles.
It said the problem concerned a small number of its millions of customers and that it expected the system to be back to normal in the next day or so.
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"British Airways has become aware of some unauthorized activity in relation to a small number of frequent flyer executive club accounts," a spokesman said.
"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."
Hackers had launched their attack "via an automated process" using information obtained elsewhere on the internet, he explained, to try to gain access to some accounts.
"We are sorry for the concern and inconvenience this matter has caused," said the spokesman.