The Guardian has admitted that up to half a million users may have been compromised by a “sophisticated and deliberate hack” of its jobs website.

However, it has moved to reassure Guardian Jobs users that it is only a “minority” who are at risk, and that its service provider had now made the site secure.

In a statement, the Guardian said it had attempted to contact everyone who may be at risk and added: “We stress our regret that this breach had occurred. This is apparently a deliberate and sophisticated crime, of which the Guardian is a victim in addition to some of our users.”

The statement posted onsite revealed that some of the data which appeared to be stolen was up to two years old, and its US sister site www.guardianjobs.com was not affected.

The police, through the central e-crime unit at New Scotland Yard, is now investigating the matter.

Guardian Jobs, along with a number of recruiters including Adecco, Kelly Services and Monster.co.uk, is a founder member of the Safer Jobs recruitment grouping, a forum created to raise awareness about, and combat, criminal activities which may be attempted on job boards and staffing companies.