The Guardian has notified around 500,000 users of its UK jobs website that their personal details may have been compromised in a hacker attack.
Users have been advised to take precautionary measures including contacting a credit reference agency and the UK fraud prevention service Cifas to ensure their information is not being used.
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The Guardian claims in reports that the "sophisticated and deliberate hack" was stopped while still in progress.
According to a security notice posted on the Guardian jobs website, administrators of the site have identified how the site was hacked and has taken steps to prevent a recurrence.
No technical details have been released because these are part of the investigation being conducted by the central e-crime unit at Scotland yard, a spokesman said.
The Guardian claims user accounts were not hacked, but compromised data could include users' names, e-mail addresses, letters and CVs stored on databases by third party supplier Madgex.
No financial or bank data was compromised and the Guardian's US-based Jobs site was not affected, the media company said.
Credit reference agencies: