Microsoft CEO Steve Ballmer has threatened Yahoo board of directors with a proxy battle if it won't agree to a buy-out in the next three weeks.

Breaking nearly two months of silence since the Yahoo board rejected the $44.6bn bid by Microsoft, Ballmer sent an open letter on Saturday to Yahoo saying Microsoft is prepared to take its offer directly to shareholders.

In blunt and harsh language, Ballmer reiterated the Microsoft opinion that its offer was "generous" and said the company had expected that a deal would be struck swiftly. "Despite this, the pace of the last two months has been anything but speedy," he wrote.

Ballmer also threatened to lower the price of the Microsoft offer if it is forced to mount a proxy battle.

"If we are forced to take an offer directly to your shareholders, that action will have an undesirable impact on the value of your company from our perspective which will be reflected in the terms of our proposal," he wrote.

Since it rejected the Microsoft offer on February 11, claiming it was too low, Yahoo has been holding out for a better offer from another company or the opportunity to strike up a partnership that would save it from agreeing to the Microsoft bid. However, Yahoo has so far been unable to negotiate another deal.

Ballmer said in his letter that the Microsoft offer is "the only alternative" to give Yahoo shareholders a fair return on their investment and input into the future of the company.

"By any fair measure, the large premium we offered in January is even more significant today," Ballmer said. "We believe that the majority of your shareholders share this assessment, even after reviewing your public disclosures relating to your future prospects."

He also accused the Yahoo executives of being unwilling to negotiate with Microsoft, and said that Yahoo stalling is wasting valuable time the combined company could be using to be more competitive in the internet business.