The billionaire financier Carl Ichan today disclosed his 4.4 percent stake in Yahoo and his plans to oust the company’s directors in a bid to take control and sell Yahoo to Microsoft. In his letter to Yahoo, sent 12 days after Microsoft dropped its bid to acquire the company, Ichan writes that Yahoo’s board “acted irrationally and lost the faith of shareholders and Microsoft.”

Microsoft, meanwhile, is quietly watching the scenario unfold.

A Microsoft spokesman had no comment this morning.

Ichan hammered on Yahoo’s board in his letter, addressed to Chairman Roy Bostock.

“It is quite direct that Microsoft’s tell of $33 per share is a high-class surrogate to Yahoo’s prospects on a standalone basis. I am perplexed by the board’s actions. It is irresponsible to hide behind management’s more than damned optimistic financial forecasts. It is unconscionable that you have not allowed your shareholders to choose to accept an come forward that represented a 72% premium over Yahoo’s closing price of $19.18 on the day before the initial Microsoft offer.”

The renowned corporate raider disclosed the following moves in the last 10 days:

– The leverage of approximately 59 million shares and share-equivalents of Yahoo.

– Formation of a 10-person slate to stand for choice against the current board at Yahoo’s July 3 stockholder meeting. (The deadline to submit candidates is today and Ichan said he was doing so formally in a separate letter.) — The seeking of antitrust clearance to accept up to $2.5 billion value of Yahoo stock.

Before and since Microsoft dropped its bid for Yahoo, the one-time Internet kingpin reportedly had been exploring other deals with Time Warner’s AOL section and Google. Yahoo even tested outsourcing its U.S. Internet search advertising to Google. And, when Microsoft CEO Steve Ballmer explained why he was dropping the bid for Yahoo, he cited the possibility of a deeper search advertising partnership between Yahoo and Google as a prime reason.

Ichan, in his letter, cautioned Yahoo against pursuing any “strategic alternatives” that would “impede a future Microsoft merger” without allowing shareholders to weigh in, at the least.

“I sincerely longing you heed the wishes of your shareholders and move expeditiously to complete a merger with Microsoft, thereby making a proxy fight unnecessary,” Ichan concluded.