It, however, noted that customers' passport and travel details were not among the information compromised by the hackers.
However, hackers stole names, addresses and payment card details of customers who used the British Airways website or mobile app between August 21 and September 5.
The airline had launched an investigation and was communicating with affected customers.
The police and relevant authorities have also been notified.
The airline's chairman and chief executive Alex Cruz said: "There was a very sophisticated, malicious criminal attack on our website. We take the protection of our customers' data very seriously", BA said, adding the airways will be contacting affected customers directly to advise them of what has happened.
British Airways contacted affected customers as soon as it found out data from its website and mobile app had been breached, its boss said on Friday.
Shares in BA's parent, International Airlines Group, fell 3 per cent in early deals on Friday due to the attack.
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Another BA customer, Melissa, 54, from Bangalore, booked a flight to Houston on August 22, a day after the airline said its system was compromised.
The penalty: If it's determined that British Airways didn't do enough to protect consumer information, it could be facing a fine of up to 4 percent of its annual revenue (that works out to about 500,000 pounds).
"Collectively, this is a blow to our privacy and British Airways joins a growing list of organisations that have faced a knock down punch".
Consumer advice website MoneySavingExpert says customers should monitor bank and credit card statements closely for signs of possible fraudulent activity.
What just happened? One of the world's largest airlines, British Airways, was hacked by cybercriminals. It also took out full page newspaper ads that included an apology.
The issue has now been resolved and the site is working properly, the company said, which has already begun to communicate with its customers who have been the victims of theft.
"As an industry until we can start making cybercrime unprofitable for adversaries they will continue to hold the cards that will yield potentially massive payouts".
Mr Cruz said at the time BA had been hit by a "major IT system failure" that caused "very severe disruption to our flight operations worldwide".