The Association of National Advertisers sent a letter to the Justice Department on Sunday, citing antitrust concerns over the proposed partnership between search leaders Yahoo and Google.

The ANA represents about 400 companies that spend about $100 billion annually on advertising. It represents and protects the interests of its members, while also advocating good advertising practices.

While on its face, the Yahoo-Google deal amounts to little more than Yahoo joining Google's vast network of advertising clients, it does give the Mountain View, Calif. company control of an overwhelming majority of the Web's search advertising inventory.

Google would not necessarily be the sole outside supplier of ads for Yahoo's pages, as the terms of their deal are non-exclusive.

According to ANA, with Yahoo in tow, Google would control up to 90 percent of all online advertising. Advertisers are arguing that allowing the partnership to occur would diminish competition, and concentrate market power under Google.

The deal will also "limit choices currently available and potentially raise prices to advertisers for high quality, affordable search advertising," the group argued further. ANA says it came to its conclusion after discussing the deal with both Google and Yahoo directly.

The Justice Department began examining the ramifications of the deal in April after the two companies' initial beta test, expanding the investigation into a formal probe in July after the deal was already announced. The DOJ has yet to draw any conclusions, though several state attorneys general have banded together to also give the deal a closer look.

A decision is expected within weeks from the DOJ, although it could be until the fall before the states reach any decision.

Yahoo expressed dismay that the ANA had came to the conclusion that it did. "Yahoo remains steadfast in its belief that this deal will strengthen Yahoo's competitive position in online advertising and will help to drive a more robust, higher quality Yahoo marketplace for our advertisers," the company said in a statement.