UK-based games developer company Codemasters has admitted that hackers have stolen names, phone numbers, date of births and email addresses of its customers following an attack on 3 June.
The company, which makes games including Operation Flashpoint, said that the security breach was "significant." However, it added that payment details were safe.
The company sent an email to all those who had registered on the site, warning them of emails and other offline enquiries asking for financial details.
The email requested 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."
Codemasters said that the information about the attack was delayed because its security team had to investigate the nature and cause of the attack. It said that the website was made offline as "as soon as the intrusion was detected".
No hacker group has yet claimed responsibility for the attack.
