The Guardian has notified 500,000 people that details they posted to the newspaper's employment site may be in the hands of hackers.
The newspaper posted a warning of the breach on its website on Friday. On Saturday, the newspaper said the system had been secured and those affected had been contacted by email.
The newspaper downplayed the impact of the breach, saying it affected "only a minority" of the 10,328,290 unique users who visit the site annually, and that some of the data lost was up to two years old.
"The police remain anxious to keep information about the apparent theft to a minimum, in order not to compromise their investigations, but did agree with us that we could inform those users who may be affected," the Guardian said.
"We stress our regret that this breach has occurred. This is apparently a deliberate and sophisticated crime, of which the Guardian is a victim in addition to some of our users."
A spokeswoman for the Metropolitan Police said the Police Central e-Crime Unit is investigating, but there have been no arrests.
The Guardian's Jobs website runs on software from a company called Madgex. Officials from the company could not immediately be reached on Monday morning.
It isn't clear what information was pilfered from the Guardian's Jobs site, but users can upload their CVs. Information in resumes and CVs could be of great use to data thieves, since those documents may contain email addresses, postal addresses, job histories and a wealth of other personal information. The data could be used for identity fraud.
In January, a database for the jobs site Monster.com was illegally hacked. The stolen data included user IDs and passwords, e-mail addresses, birth dates, gender, ethnicity, and in some cases, users' states of residence in the US. No resumes or Social Security numbers were lost.
Monster was also the target of a massive data breach two years ago. Hackers obtained the log-in credentials for companies seeking employees then accessed Monster's database, sending up to 1.6 million records to a remote server.