EasyJet has admitted that a “highly sophisticated cyber-attack” has affected approximately nine million customers.
It said email addresses and travel details had been stolen and that 2,208 customers had also had their credit and debit card details “accessed”.
The firm has informed the UK’s Information Commissioner’s Office while it investigates the breach.
EasyJet first became aware of the attack in January.
It told the BBC that it was only able to notify customers whose credit card details were stolen in early April.
“This was a highly sophisticated attacker. It took time to understand the scope of the attack and to identify who had been impacted,” the airline told the BBC.
“We could only inform people once the investigation had progressed enough that we were able to identify whether any individuals have been affected, then who had been impacted and what information had been accessed.”
Stolen credit card data included the three digital security code – known as the CVV number – on the back of the card itself.