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.
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: