British Airways says some frequent flier accounts have been hacked, but the airline maintains most personal information is safe.
The London-based airline did not say how many accounts were compromised, but said they have been locked down and can no longer be accessed.
British Airways is not aware of any access to any personal information within Executive Club accounts, including travel histories or credit card details, according to spokeswoman Michele Kropf.
Customers were emailed details on how to reactivate their accounts.
The breach apparently was the result of a third party using information obtained elsewhere on the internet, via an automated process, to try to gain access to accounts.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
It follows problems for hotel companies Hilton and Starwood as well as United and American airlines.
"It's a major cost to the programs themselves. Members are always made whole and do not lose their points once the problem is identified," said Gary Leff, co-founder of online frequent flier discussion site MilePoint.
"The best things you can do to protect your accounts are to use unique passwords ... and remain vigilant. Check your accounts daily so that you notice miles disappearing quickly."
