The Google-Yahoo! deal announced in June is the subject of an investigation by the US Department of Justice (DoJ) on competition grounds.
ANA chief executive Bob Liodice said that it had written to the DoJ objecting to the deal.
"The letter, authorized by the ANA Board, notes that a Google-Yahoo partnership will control 90 percent of search advertising inventory," said Liodice in a staement.
Yahoo! is one of Google's major competitors in the market for online advertising but announced earlier this year that it would allow Google to supply it with some search-related advertising content. The move was predicted to boost Yahoo!'s short term revenues but at the expense of handing greater market control to Google.
The ANA said that the deal would harm its members' ability to buy advertising cheaply from a range of suppliers.
"[The letter] states ANA's concerns that the partnership 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," said Liodice's statement.
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