A consumer group says a plan by Google to share advertising with rival Yahoo will create a monopoly that threatens privacy.

Many advertisers also oppose the deal that they fear will drive up costs.

US Public Interest Research Group (PIRG) says advertisers competing with Google and Yahoo, which together have more than 80% of the search-advertising market, would have to get more information on internet users as they would be unable to compete with the market leaders on price.

The result would be a loss of privacy for consumers, PIRG said.

"The proposed agreement induces the remaining paid search-advertising outlets to resort to privacy-invasive techniques which harms consumer privacy online," PIRG said in a letter to US Attorney General Michael Mukasey.

The revenue-sharing agreement, which had its genesis when Yahoo was fighting off Microsoft's advances, is expected to net Yahoo $800 million (£499.24 million) a year.

"Google's success must not be allowed to morph into monopoly power," PIRG said.

Google has sometimes been accused of a cavalier attitude to privacy, particularly over its "StreetView" in Google Earth and Google Maps.

To combat these charges, Google joined Yahoo and Microsoft this week to release "The Global Network Initiative", described as a "first step" to better safeguarding free speech and online privacy rights.

The companies spent 18 months consulting human rights officials, academics and ethical investment groups. But Marc Rotenberg, director of the Electronic Privacy Information Center, says that self-regulatory proposals allow firms to "interpret them as they wish or back out of them when they choose".