Oct. 19 (Bloomberg) -- Microsoft Corp. and Yahoo! Inc., whose Internet-search agreement is under antitrust review by the U.S. Justice Department, received backing from the biggest advertising agencies and a trade group that represents them.

Chief executive officers from WPP Plc, Interpublic Group of Cos., Publicis SA and Omnicom Group Inc. signed a letter to the Justice Department saying the combination “is good for advertisers, marketing services agencies, Web site publishers and consumers.” The group asked regulators to bring their review to “a speedy conclusion.”

The companies need to convince the agency, which last year scuttled a search agreement between Yahoo and market leader Google Inc., that the deal doesn’t constrain competition. Microsoft said last month that the Justice Department had expanded its review of the deal, in which Yahoo agreed to use Microsoft’s Bing search engine on its Web sites.

The letter came from Nancy Hill, CEO of the American Association of Advertising Agencies, and was posted on the group’s Internet site.

The 10-year agreement also would have Yahoo selling ads next to search results from both companies. Microsoft and Yahoo are arguing their combination is needed to create a viable competitor to Google.

Market Shares

Microsoft, based in Redmond, Washington, fell 14 cents to $26.36 at 4 p.m. New York time on the Nasdaq Stock Market. Yahoo, based in Sunnyvale, California, added 41 cents to $17.22. Google, in Mountain View, California, rose $2.24 to $552.09.

Google had 64.9 percent of U.S. Internet search traffic in September, according to research firm ComScore Inc. in Reston, Virginia. Yahoo and Microsoft had 28.2 percent combined. Outside the U.S., Google has an even bigger lead.

In November 2008, Google walked away from a planned search deal with Yahoo after the Justice Department said it would sue to stop the alliance. That agreement was opposed by the Association of National Advertisers, a trade group that represented companies like Wal-Mart Stores Inc. and General Motors Corp.

To contact the reporter on this story: Dina Bass in Seattle at dbass2@bloomberg.net

Last Updated: October 19, 2009 16:13 EDT