SAN FRANCISCO – On Thursday, Yahoo! Chairman Roy Bostock and Chief Executive Jerry Yang of Yahoo! sent a letter to shareholders calling Microsoft’s actions “stupefying”, and tearing into billionaire activist investor Carl Icahn, who has been working with Microsoft to push its proposed takeover, for his lack of knowledge of the Internet business.

Yahoo!�??s annual meeting is Aug. 1, when Icahn will launch his proxy fight to replace the Yahoo! board with one of his own choosing, that will do his bidding.

Thursday’s letter, reported by Forbes magazine, also said, “Icahn has made it clear that his only objective is to sell part or all of Yahoo! to Microsoft. That fact, combined with his lack of an operating plan going forward, means that he will have no leverage to negotiate a fair deal with Microsoft.”

“Microsoft’s flip flops and inconsistencies over the past five months are so stupefying that one can only conclude that Microsoft was never fully committed to acquiring Yahoo!” the letter said.

Thursday’s words are the latest round in the lengthy struggle over ownership of the web portal, and comes only days after Yahoo! and Microsoft tried–and failed–to negotiate a transaction in which the software giant acquires the Web portal’s search business. The terms are complex, but would involve splitting the company, with Microsoft taking the search engine. Yahoo! issued a statement over the weekend, saying Microsoft’s latest proposal, issued late last Friday, isn’t in the best interest of shareholders.

Icahn, who owns about 5.0 percent of Yahoo!, followed the weekend rebuttal with a sharpened focus on replacing the company’s board by filing the final nominating papers for a slate of candidates that will oppose Yahoo!’s current nine directors an Aug. 1 shareholder vote. In his own letter to shareholders, Icahn accused the incumbent board members of being more interested in protecting their jobs than evaluating the merits of Microsoft’s latest offer for Yahoo’s online search operations.

For all the name calling, Aug. 1 will be the date when shareholders can chose between hoping Yahoo! can somehow turn itself around, or selling to Microsoft, taking their gains and moving on.

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