Microsoft CEO Steve Ballmer threatened Yahoo with a proxy war over the weekend, setting a three week deadline for Yahoo to come to an agreement on its takeover bid, Editor & Publisher reported Monday.

“If we have not concluded an agreement within the next three weeks, we will be compelled to take our case directly to your shareholders, including the initiation of a proxy contest to elect an alternative slate of directors for the Yahoo board,” Ballmer stated in a letter on Microsoft's Web site.

Ballmer also stated that his Jan. 31 offer of US$44.6 billion, or $31 per share, is more generous than it was when the offer was made, as Yahoo's stock price has been decreasing, E&P reported.

Yahoo CEO Jerry Yang and Chairman Roy Bostock responded that Microsoft's offer continues to undervalue Yahoo.