The incident is being investigated by the police
Computer hackers have targeted the Guardian newspaper's jobs website in a "sophisticated and deliberate" move, the company has said.
The breach put the personal details of some of the site's users at risk, and those who may have been affected have been identified and e-mailed.
The Guardian said it had since been "assured" by the supplier that runs the site that the system was now secure.
Officers from the Metropolitan Police's e-crime unit are investigating.
The Guardian says its jobs site attracts more than two million unique users a month.
The company e-mailed those affected, saying: "You have used the site to make one or more job applications and we believe your personal data, relating to those applications, may have been accessed."
It said there was "no reason to believe" financial or banking data was compromised, but passed on police advice about taking precautions, such as contacting creditors and asking them to monitor users' accounts.
Recipients of the e-mail were also given the details of several organisations offering advice and services on identity fraud.
A Guardian technology director said: "Not all users are affected by any means."
But one user Paul Rocks said he was angry about what had happened.
"It's very disconcerting to think that some very important details with regards to my identity could be in the wrong hands. These include my past employment details, date of birth and my current address.
"I'm also annoyed that the responsibility for doing something about this seems to have been passed on to me."
Mr Rocks, 40, who is a freelance journalist in London, said the details a hacker could have obtained would be enough for an identity thief to apply for a loan or credit card.
He has contacted his bank, who told him they would pass on a note of the incident to a credit reference agency.
The hack does not affect the separate US site, which is independent of the UK operation.
A subsequent statement on the jobs website said: "We would like to assure you that we are absolutely committed to protecting the privacy of our users and we are treating this situation with the utmost seriousness."