In a move aimed at speeding up the purchase of Yahoo, Microsoft on Saturday wrote a letter to the Yahoo board of directors threatening to take its proposal directly to its shareholders if an agreement is not reached in the next three weeks. Yahoo, however, has once again rejected the bid saying it “substantially undervalues” the company.

Yahoo’s CEO Jerry Yang said the company is still open to a deal, but not at the $31 per share offered by Microsoft, which represented a 62% premium over the company’s closing price of $19.18 January 31. Yahoo’s stance could make sense considering the company’s shares have been rising since the Microsoft bid was first announced, closing at $28.36 apiece Friday.

Yang also accused Ballmer of mischaracterizing the state of discussions between the two companies over a possible deal and described the threat of a proxy battle as being “counterproductive and inconsistent” with the objective of a friendly transaction. It’ll be interesting to see what Microsoft’s response will be.