British Airways has notified the police after around 380,000 customer payment cards were compromised.
The airline said that theft of customer data from its website and mobile app had occurred.
BA reported on Thursday that the personal and financial details of customers who made bookings on its website or app from 10.58pm on August 21 until 9.45pm on September 5 had been compromised.
- Business analysis by Joanna Partridge
BA said the stolen data did not include travel or passport details, adding that it was investigating the security breach as a matter of urgency.
The company said the breach had been resolved and the website was now working normally.
Alex Cruz, British Airways’ chairman and chief executive, told ITV News the hack had been a "malicious" attack.
He said: "We are deeply sorry for the disruption that this criminal activity has caused. We take the protection of our customers' data very seriously."
The company said it is communicating with affected customers and advised anyone who believed they may have been affected to contact their banks or credit card providers.