Currently, investigations are in progress but the airline maintains there is no evidence that "any personal information of any nature has been misused".
EasyJet said it first became aware of the attack in January.
While there's no evidence the information has been misused, the airline said it has closed off the unauthorized access and will contact affected customers no later than May 26.More news: NASCAR at Darlington: Chase Elliott flips off Kyle Busch after late spin
Johan Lundgren, EasyJet chief executive, said: "We take the cybersecurity of our systems very seriously and have robust security measures in place to protect our customers' personal information".
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EasyJet says email addresses and travel details have been obtained by the infiltrator, and it's promised to contact the customers affected. EasyJet has now chose to go public in order to reach all the nine million customers affected.
Once EasyJet learned of the attack, they notified the UK's National Cyber Security Centre and the ICO.More news: Trump dismisses FDA warnings about hydroxychloroquine
Affected customers exposing their credit card details are advised to block the affected cards and request a new one from their respective financial institution, and always keep a close eye on your bank and payment card statements for any unusual activity and report to the bank if you find any. It said all nine million passengers would be contacted with advice regarding protective steps to minimize the risk of potential phishing.
The airline said: "We are advising customers to continue to be alert as they would normally be, especially should they receive any unsolicited communications".
Following a similar data breach at British Airways in 2018, some found this a frustrating and time-consuming task.
Easyjet has been going through a torrid time, with the novel coronavirus forcing it to shut down flying operations completely as of 16 April.More news: Jason Momoa and Peter Dinklage in talks to reteam for vampire movie
That campaign is due to come to a head at a corporate extraordinary general meeting this Friday, 22 May. Shareholders will vote on these proposals at the Friday meeting.