However, it said that payment details were not compromised.
The latest security breach comes in the midst of a spate of hacker attacks, including several against Sony.
Codemasters said it took the compromised website offline as "as soon as the intrusion was detected".
A subsequent investigation revealed that hackers managed to take the personal details of thousands of users, including names, addresses, email addresses, phone numbers and dates of birth, passwords, IP addresses, XBox gamer tags, and biographies.
In an e-mail sent to some of its customers, the firm advised users to "change any passwords you have associated with other Codemasters accounts.
"If you use the same login information for other sites, you should change that information too.
"Furthermore, be extra cautious of potential scams, via email, phone, or post that ask you for personal or sensitive information."
A spokesperson for Codemasters told BBC News that they still had no idea who targeted their sites, or how many people had their details compromised, although they said that it would affect tens of thousands of users.
Codemasters said its website - codemasters.com - would remain offline "for the foreseeable future" with users being directed to its Facebook page.
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