It's official: Microsoft is deploying lobbyists to oppose Google's proposed purchase of DoubleClick, which it claims raise "serious competitive issues" in the online-ad space.

According to a recent public disclosure filing with the US Senate, the software maker has retained veteran lobbyists Thomas Boggs and Kathleen Ireland, along with Antitrust Modernization Commission vice chairman and former Clinton White House attorney Jonathan Yarowsky. All of them work for the prominent law firm Patton Boggs.

Their charge, the filing said is "Competitive issues surrounding Google-DoubleClick merger".

As the Federal Trade Commission continues to weigh whether the the US$3.1 billion deal passes antitrust muster, Microsoft has made no secret of its concerns, which centre on claims that the merger raises serious competitive questions in the online-ad space.

Google, meanwhile, has repeatedly said it's confident that the acquisition will benefit consumers and that the threat from its rival can be contained.

Although the filing is marked as received on August 9, the "effective date" of the lobbyists' registration is actually May 15. That's around the same time Google disclosed that it had picked up four new lobbyists, including a former high-ranking Department of Justice antitrust lawyer, to help make the case for its buy. Neither company has agreed to talk in more detail about its lobbying efforts.

It wasn't immediately clear whether the timing of Microsoft's latest lobbying filing has anything to do with sealing its own ad buy--a $6 billion takeover of the firm aQuantive -- last Friday. That deal had already cleared an antitrust waiting period with federal regulators.

The Associated Press first noted Microsoft's new disclosure earlier this week.