Facebook has admitted that it secretly hired a public-relations group in the US with the aim of generating stories critical of Google's approach to privacy.
The disclosure is the latest sign of the increasing rivalry between Facebook and Google , as they go head-to-head over internet users' time and advertisers' budgets.
Burson-Marsteller, a WPP-owned PR agency whose clients also include Microsoft , contacted US newspaper reporters and opinion-piece writers with a view to securing coverage on Google's alleged use of personal information from Facebook and other social networks.
The agency did not inform the journalists that it was acting on behalf of Facebook.
The PR company said: "Whatever the rationale, this was not at all standard operating procedure and is against our policies, and the assignment on those terms should have been declined." It said it was no longer working with Facebook.
Both Facebook and Google have come under fire from privacy campaigners for the amount of personal data they collect and retain online.
Facebook denied that it had intended to run a "smear" campaign over Social Circles, a little-known Google feature that charts connections between internet users based on their profiles on sites such as Facebook, Twitter and LinkedIn.
Both Facebook and its former agency said that the information sent to the media was publicly available and independently verifiable. Facebook said it took the anonymous approach because it wanted independent reporters to reach conclusions about the service.
"The issues are serious and we should have presented them in a serious and transparent way," Facebook said.
Google declined to comment.
Rosanna Fisk, chief executive of the Public Relations Society of America, said the activity was "unethical and improper".
"This is a PR nightmare you have to disclose where you stand on things," she said.
The misstep comes at a time when Facebook is seeking to raise its profile in Washington and is preparing for a likely stock market debut next year.
Simon Davies, director at Privacy International, a campaign group that has been critical of both Facebook and Google said: "Facebook should be taking the best advice to find a way through its own problems, not exploit those of other companies ... Once a company resorts to this sort of tactic it has already lost the plot."
A Facebook spokesman said: "There were two issues: one is the commercial issue for Facebook and the other is the privacy issue for Facebook users. We wanted to distinguish the two."
It did not comment further on its commercial concerns over Google's service.