The Association of National Advertisers said on its Web site that the letter to Thomas Barnett, assistant attorney general in charge of the Justice Department's antitrust division, came after a "comprehensive, independent analysis" and meetings with Google and Yahoo executives.

The ANA did not disclose the text of the letter but said it states its concern that "a Google-Yahoo partnership will control 90 percent of search advertising inventory and ... 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."

The ANA says it represents 400 companies - including Apple Inc., The Coca-Cola Co., Exxon Mobil Corp., Proctor & Gamble Co. and General Motors Corp. - with 9,000 brands.

The Justice Department and several state attorneys general are examining the partnership between Yahoo and Google, which would allow Google to sell some of the ads displayed alongside search results on Yahoo's Web site. Yahoo and Google have insisted the deal would benefit consumers and advertisers.

U.S. lawmakers, as well as consumer and civic groups, have also voiced concern over the deal.

Yahoo embraced the partnership with its rival as an alternative to a $47.5 billion acquisition offer from Microsoft Corp.