British Airways announced that it was probing a massive theft of customer data from its website and mobile app.
The airline said the breach took place between 10.58 p.m. on August 21 and 9.45 p.m. on September 5.
"The breach has been resolved and our website is working normally," it said in a statement on Thursday night.
"We have notified the police and relevant authorities. We are deeply sorry for the disruption that this criminal activity has caused. We take the protection of our customers’ data very seriously."
British Airways said personal and financial details of customers making bookings had been compromised.
About 380,000 transactions were affected, but the stolen data did not include travel or passport details.
All affected customers had been contacted on Thursday night.
Alex Cruz, British Airways’ Chairman and Chief Executive, said: "We are deeply sorry for the disruption that this criminal activity has caused. We take the protection of our customers’ data very seriously."
This is not the first customer relations problem to affect the airline in recent times, the BBC said.
In July, the airline apologised after IT issues caused dozens of flights in and out of London’s Heathrow Airport to be cancelled.
In June, more than 2,000 British Airways passengers had their tickets cancelled because the prices were too cheap.
In May 2017, serious problems with British Airways’ IT systems led to thousands of passengers having their plans disrupted, after all flights from Heathrow and Gatwick were cancelled.>