The data breach, which BA reported to the ICO in September last year, involved the airline’s user traffic being diverted to a fraudulent website, where customer details were harvested.

According to the ICO, the personal data of around 500,000 customers was affected by the incident. The ICO has issued a notice of its intention to fine for infringements of the General Data Protection Regulation (GDPR).

The data included personal and financial details of customers making bookings and changes on the airline’s website and app.

The ICO said information was compromised by "poor security arrangements" at BA. 

BA has co-operated with the ICO investigation and made improvements to its security arrangements, the ICO said. The airline has a chance to make representations before the fine is finalised.

Alex Cruz, British Airways chairman and chief executive said the company was "surprised and disappointed" by the ICO’s findings, adding that the airline had found no evidence of "fraud or fraudulent activity" on accounts linked to the breach. 

Willie Walsh, chief executive of BA parent company IAG, said: "British Airways will be making representations to the ICO in relation to the proposed fine. We intend to take all appropriate steps to defend the airline’s position vigorously, including making any necessary appeals."

Elizabeth Denham, information commissioner, said: "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. That’s why the law is clear – when you are entrusted with personal data you must look after it. Those that don’t will face scrutiny from my office to check they have taken appropriate steps to protect fundamental privacy rights."