The Justice Department and several state attorneys general are still examining the partnership between Yahoo and Google. The two companies had voluntarily delayed implementation of this deal for 100 days until regulators clear up the review, though they have said they are not required to wait for the regulatory approval.

WSJ: “Whether the letter will influence federal antitrust regulators remains unclear, but it is considered a blow to Yahoo and Google because of the trade group’s high profile. Until now, big marketers have been reluctant to come out against the deal publicly because of Google’s growing power in the ad business.”

Meanwhile, both Yahoo and Google have reacted late today on this letter. Yahoo: The “prices are determined by advertiser demand-driven auctions, not by collaboration between Yahoo and Google…will help to drive a more robust, higher quality…marketplace for our advertisers”. From Google: “While some have raised questions about the agreements’ potential impact on ad prices, advertisers care far more about getting a good return on their advertising dollar than they do about buying cheap ads that don’t bring in customers, and this agreement will clearly help advertisers reach Yahoo users more efficiently.”