A cyber attack on UK airline EasyJet accessed the email and travel details of around nine million customers, as well as the credit card details of more than 2,000 of them, it said.
The news of the late January attack means the budget airline, which has grounded most of its flights due to the Covid-19 pandemic and is locked in a long-running battle with its founder and biggest shareholder, could face a hefty fine.
British Airways, which was hit in 2018 with the theft of hundreds of thousands of credit card details, is still appealing a fine from the Information Commissioner's Office (ICO) of £183.4m.
"Since we became aware of the incident, it has become clear that owing to Covid-19 there is heightened concern about personal data being used for online scams," the airline's chief executive Johan Lundgren said.
"As a result, and on the recommendation of the ICO, we are contacting those customers whose travel information was accessed and we are advising them to be extra vigilant, particularly if they receive unsolicited communications," he added.
EasyJet said it did not look like any personal information had been misused. It has engaged leading forensic experts to investigate the issue and has also notified the National Cyber Security Centre.
"We would like to apologise to those customers who have been affected by this incident," Mr Lundgren said.
Hackers around the world have stepped up their efforts in recent months, taking advantage of the pandemic to trick people into revealing their passwords and other sensitive data. Government officials have also warned of greater risks as people working from home create opportunities for hackers.
EasyJet shares are down 64% in the last three months.