British Airways has been fined £20 million for failing to protect data that left more than 400,000 of its customers’ details the subject of a 2018 cyber attack.

The fine is the biggest such penalty to be imposed by the data protection watchdog till date.

The Information Commissioner’s Office (ICO) said its investigators found BA should have identified weaknesses in its security and resolved them with measures available at the time, which would have prevented the data breach.

“Their failure to act was unacceptable and affected hundreds of thousands of people, which may have caused some anxiety and distress as a result,” the ICO said.

BA said in a statement that it had alerted customers as soon as it became aware of the attack.

The penalty was considerably less than the 183.4 million pounds the ICO proposed last year – in part reflecting the crisis the airline industry is now facing due to COVID-19.

Still, shares in BA’s Anglo-Spanish parent IAG ICAG.L slid to session lows following the announcement. By 0917 GMT, they were 3% lower at 93.2 pence.

On Monday, IAG announced it was replacing BA’s chief executive Alex Cruz with Aer Lingus boss Sean Doyle with immediate effect.

Announcing the penalty, the regulator said its investigators found that BA did not detect the attack on June 22, 2018 – but was alerted by a third party more than two months later, on Sept. 5.

The ICO added that it was not clear whether or when the company would have identified the attack itself.

“This was considered to be a severe failing because of the number of people affected and because any potential financial harm could have been more significant,” it said.

Explaining why the final penalty was substantially lower than first suggested, the regulator said it considered representations from BA and the economic impact of the coronavirus pandemic, which has upended the travel industry.

“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,” BA said in a statement.

Other major cyber incidents in the recent past include another London-listed airline, easyJet, which earlier this year said hackers had accessed the email and travel details of around nine million customers.

U.S. hotel operator Marriott International in March suffered its second data incident in less than two years, with information of about 5.2 million hotel guests suffering a breach.