Budget airline easyJet has said it has been the target of a "highly sophisticated" cyber attack.
Around nine million customers have had their email addresses and travel details accessed, and around 2,208 had their credit card details accessed.
The company said customers will be contacted in the next few days.
It added: "If you are not contacted then your information has not been accessed. Other than as referenced in the following paragraph, passport details and credit card details of these customers were not accessed."
They said that action has already been taken to contact all of the customers whose credit cards have been accessed and "they have been offered support".
The company added: "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 (Information Commissioner's Office), 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.
"We are advising customers to continue to be alert as they would normally be, especially should they receive any unsolicited communications. We also advise customers to be cautious of any communications purporting to come from easyJet or easyJet Holidays.
"We're sorry that this has happened, and we would like to reassure customers that we take the safety and security of their information very seriously."
EasyJet is in the process of contacting the relevant customers directly and affected customers will be notified no later than 26(th) of May.
Chief Executive Johan Lundgren said: "We take the cyber security of our systems very seriously and have robust security measures in place to protect our customers' personal information. However, this is an evolving threat as cyber attackers get ever more sophisticated.
"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. 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: "Every business must continue to stay agile to stay ahead of the threat. We will continue to invest in protecting our customers, our systems, and our data.
"We would like to apologise to those customers who have been affected by this incident."
Customers can also find further advice at www.actionfraud.police.co.uk
