Microsoft's top lawyer on Thursday warned that any deal between Yahoo and Google would hurt competition.
"Any definitive agreement between Yahoo and Google would consolidate over 90 percent of the search-advertising market in Google's hands. This would make the market far less competitive, in contrast to our own proposal to acquire Yahoo," Microsoft General Counsel Brad Smith said in a statement.
The statement follows reports that Yahoo is considering a limited test of using Google to deliver some search advertising.
Smith said Microsoft will "assess closely all of our options."
Microsoft proposed on February 1 to buy Yahoo for $31 a share, but it has thus far been rebuffed by Yahoo, though the two sides have had a couple preliminary meetings. On Saturday, Microsoft set a three-week deadline for Yahoo to come to the negotiating table, or it would nominate its own slate of directors.
"Our proposal remains the only alternative that offers the shareholders full and fair value for their shares, gives every shareholder a vote on the future of the company, and enhances choice for content creators, advertisers, and consumers," Smith said.