British Airways has is facing a fine of £20m for failing to detect a hack for more than two months which meant details of more than 400,000 were exposed.
Investigators found the airline should have identified the security weaknesses which enabled the attack to take place, according to the Information Commissioner's Office.
Information Commissioner Elizabeth Denham said: "People entrusted their personal details to BA, and BA failed to take adequate measures to keep those details secure.
"Their failure to act was unacceptable and affected hundreds of thousands of people, which may have caused some anxiety and distress as a result.
"That's why we have issued BA with a £20 million fine - our biggest to date.
"When organisations take poor decisions around people's personal data, that can have a real impact on people's lives.
"The law now gives us the tools to encourage businesses to make better decisions about data, including investing in up-to-date security."
The ICO announced in July last year that British Airways could be fined more than £183 million.
This is more than nine times the £20 million the airline has eventually been fined.
The ICO said it considered "representations from BA and the economic impact of Covid-19 on their business" before setting the final penalty.
A British Airways spokeswoman said: "We alerted customers as soon as we became aware of the criminal attack on our systems in 2018 and are sorry we fell short of our customers' expectations.
"We are pleased the ICO recognises that we have made considerable improvements to the security of our systems since the attack and that we fully co-operated with its investigation."
The attacker is believed to have potentially accessed the personal data of approximately 429,612 customers and staff.
This included the names, addresses, payment card numbers and the three digits on the back of cards of 77,000 customers, and card numbers only for 108,000 customers.
Usernames and passwords of British Airways' employee and administrator accounts, as well as the usernames and PINs of up to 612 of the airline's Executive Club accounts, were also potentially accessed.