The US Association of National Advertisers (ANA) has written to the Department of Justice (DoJ) to voice concerns about a Google-Yahoo search advertising partnership.

The two internet firms announced the deal in June this year.

At that time Omid Kordestani, senior vice president of global sales and business development at Google, said in a blog post: "Why did we make this agreement? Quite simply, we think it is good for users, advertisers and publishers.

"By offering Google's industry-leading technology to Yahoo, the whole system becomes more efficient, and everyone benefits."

As the DoJ debates the recently announced partnership, Bob Liodice, president and chief executive at the ANA, said that the group had sent a letter to Thomas Barnett, assistant Attorney General at the DoJ, in which it cites its objections.

Liodice said that the letter was the result of independent analysis, and included input from board members and face-to-face discussions with both parties.

"The letter, authorised by the ANA board, notes that a Google-Yahoo partnership will control 90 per cent of search advertising inventory, and states ANA's concerns that the partnership will likely diminish competition, increase concentration of market power, limit choices currently available and potentially raise prices to advertisers for high quality, affordable search advertising," it states.

Yahoo has already responded to the letter. "We are disappointed with the ANA board's position regarding Yahoo's non-exclusive search marketing agreement with Google," said the company.

"Yahoo remains steadfast in its belief that this deal, in which prices are determined by advertiser demand-driven auctions, and not by collaboration between Yahoo and Google, will strengthen Yahoo's competitive position in online advertising and will help to drive a more robust, higher quality Yahoo marketplace for our advertisers."