The photo has been used for illustrative purposes.

ICO proposed the fine on Monday, months after BA revealed it had been the victim of a hack. The scam saw customers diverted to a fake website where credit card details were harvested by the attackers.

The ICO proposed a penalty of £183.4 million, or 1.5% of British Airways' 2017 worldwide turnover, for the hack, which it said exposed poor security arrangements at the airline.

It comes about a year after European Union member states began implementing the most sweeping change in data protection rules in a generation.

"People's personal data is just that - personal. When an organisation fails to protect it from loss, damage or theft it is more than an inconvenience," Information Commissioner Elizabeth Denham said. "That's why the law is clear - when you are entrusted with personal data you must look after it."

The General Data Protection Regulation, or GDPR for short, is designed to make it easier for EU residents to give and withdraw permission for companies to use personal information — but also forces companies that hold data to be accountable for looking after it. Authorities can fine companies up to 4% of annual revenue or 20 million euros ($22.5 million), whichever is higher, for breaching the rules.

The Information Commissioner's Office says its investigation of BA found that "poor security arrangements" compromised login, payment card, and travel booking details as well as name and address information.

The parent company of BA, International Airlines Group, said it would fight the proposed fine. It has 28 days to make its case in the first step of the process, which could take some time to complete.

"We intend to take all appropriate steps to defend the airline's position vigorously, including making any necessary appeals," said IAG CEO Willie Walsh.