The pending Internet search deal between Microsoft Corp. (MSFT) and Yahoo Inc. (YHOO) has got the backing of four of the world's largest advertising firms, likely boosting the deal's chances of securing antitrust approval.

The chief executives of the WPP Group PLC (WPP.LN), Publicis Groupe SA (PUBGY), the Interpublic Group of Cos. (IPG) and Omnicom Group Inc. (OMC) signed an open letter from the American Association of Advertising Agencies, saying the deal "enhances" competition and calling on the U.S. Justice Department to clear it.

Microsoft and Yahoo in July agreed to work together on a broad search advertising partnership to better combat market leader Google Inc. (GOOG) The pact, if cleared, will reduce the number of significant competitors in the Internet search market to two.

"We believe that Yahoo and Microsoft's proposal to combine their technologies and search platforms is good for advertisers, marketing service agencies, website publishers and consumers," the letter, from the AAAA President and CEO Nancy Hill, said.

"These benefits are too important to wait for," it added.

The agreement of the four CEOs - Martin Sorrell of WPP Group, Maurice Levy of Publicis, John Wren of Omnicom and Michael Roth of Interpublic - will likely be persuasive to the Justice Department, which is reviewing the deal.

Collectively, these companies speak for the bulk of the world's advertising agencies. They are notorious for their inter-company rivalry and general inability to agree on anything.

The Justice Department in September said that the deal was entering a so-called "second request" phase, which indicates that it requires more detailed scrutiny.

It remains unclear whether European Union regulators will launch a formal antitrust review of the deal, although they have been in discussion with Microsoft and Yahoo.

Although the European Union typically scrutinizes Microsoft's activities closely, Google's market share in Europe is such that legal experts don't expect authorities to block the deal. In most European countries Google's share of the Internet search market is well over 90%, higher than in the U.S.

"We welcome the broad industry support and we're still hopeful that a deal will close some time early in 2010," a Microsoft spokesman said.

-By Jessica Hodgson, Dow Jones Newswires; 415-439-6455; jessica.hodgson@dowjones.com