California ? Microsoft Chief Executive Steve Ballmer sent a letter to Yahoo’s board of directors Saturday threatening to take the company’s offer to buy Yahoo for $31 a share directly to shareholders if directors don’t reach a deal within the next three weeks.

And if there’s no deal, Ballmer added, Microsoft will cut its offering price.

“It has now been more than two months since we made our proposal to acquire Yahoo! at a 62 percent premium to its closing price on January 31, 2008, the day prior to our announcement,’ Ballmer wrote.

“Our goal in making such a generous offer was to create the basis for a speedy and ultimately friendly transaction. Despite this, the pace of the last two months has been anything but speedy.”

While the Yahoo board stalled, the value of the Sunnyvale, Calif., Internet company declined along with a falling stock market and weakening economy, Ballmer asserted. He also noted that data from independent market research firms and other indicators suggest Yahoo’s share of Internet search queries and page views has declined.