Microsoft Corp., whose $44.6 billion takeover offer was spurned by Yahoo! Inc., said the Internet company’s directors risk facing a proxy battle and a lower bid if they fail to agree to terms in three weeks.

If the directors refuse to negotiate, Microsoft plans to nominate a board slate and take its case to investors, the chief executive officer, Steven Ballmer, said Saturday in a statement. He suggested the deal’s value might decline if Microsoft takes the bid directly to them.

Mr. Ballmer said the companies have had no “meaningful” talks in the two months since the bid, a 62% premium to the stock price at the time. His stance may force the chief executive officer of Yahoo, Jerry Yang, to start discussions to avoid a fight with the world’s biggest software maker.

“I think they’re surprised that Yahoo’s board has strung this out this long,” chief investment manager at Becker Capital Management in Portland, Ore., Patrick Becker, said in an interview. “If they have to go to a proxy fight, they possibly will lower the bid to compensate themselves for the time.” Mr. Becker manages Microsoft shares among its $2.4 billion in assets.

Combining with Yahoo would allow Microsoft to unite the second- and third-most popular search engines in America, helping them to take on Google, which gets more than half the Internet queries in the country.

Yahoo rejected the $31-a-share cash-and-stock bid on February 11, saying the price didn’t reflect its value. Making Microsoft go to Yahoo investors “will have an undesirable impact on the value of your company from our perspective, which will be reflected in the terms of our proposal,” Mr. Ballmer said in the statement, which included a letter to Yahoo shareholders. A spokeswoman for Sunnyvale, Calif.- based Yahoo, Tracy Schmaler, said she couldn’t immediately comment.

Yahoo said in February that it would seek alternatives to the Redmond, Wash.-based company’s bid that would appeal to shareholders. The company has held talks with Time Warner Inc. and News Corp. about partnerships, people with knowledge of those discussions said in February.

Yahoo fell 87 cents to $27.49 in extended trading on April 4 after Reuters reported Microsoft was re-evaluating its bid. The company closed at $28.36 on the Nasdaq Stock Market. Microsoft closed at $29.16, valuing the bid at about $29.36 a share.

In the two months since Microsoft’s first advances, the American economy has deteriorated, affecting the business of Internet companies, Mr. Ballmer said. Yahoo and Microsoft’s share of American searches dropped in February from the previous month as more Web users chose Google Inc., according to Reston, Va.-based researcher ComScore Inc.

Last month, Yahoo pointed to its no. 2 position in Web search, its operations in Asia and the potential cost savings of the deal to show it’s worth more than Microsoft’s offer. Yahoo said then that sales will climb at least 19% in each of the next two years and that growth would be higher than analysts anticipated.

Sanford C. Bernstein analyst Jeffrey Lindsay in New York called Yahoo’s growth predictions in the next two years “too bullish” given the American slowdown.

“Public indicators suggest that Yahoo’s search and page view shares have declined,”Mr. Ballmer said. “By any fair measure, the large premium we offered in January is even more significant today. We believe that the majority of your shareholders share this assessment, even after reviewing your public disclosures relating to your future prospects.”

Yahoo hasn’t announced the date of its shareholders meeting. All 10 of its directors are up for reelection. The last meeting occurred June 12, and under Delaware law, the company must hold one every 13 months.

“It’s only a question of time before Microsoft takes them over,” Mr. Lindsay said April 4. “It’s just unfortunate that Yahoo has dragged it out so long because they are probably going to miss a few dollars that they could have obtained if they had negotiated immediately.”