EasyJet has admitted that a "highly sophisticated cyber-attack" has affected nine million customers.
It said email addresses and travel details had been stolen and that 2,208 customers had also had their credit card details "accessed".
The firm has informed the UK's Information Commissioner's Office and is continuing to investigate the breach.
EasyJet said it first became aware of the attack in January.
In a statement it said: "We take issues of security extremely seriously and continue to invest to further enhance our security environment.
"There is no evidence that any personal information of any nature has been misused, however, on the recommendation of the ICO, we are communicating with the approximately nine million customers whose travel details were accessed to advise them of protective steps to minimise any risk of potential phishing."