SAN FRANCISCO - Yahoo Inc. chief executive officer Jerry Yang spent months fending off Microsoft Corp.'s unsolicited takeover bid. Now he may only have a few weeks to persuade the software-maker to revive its last offer of $47.5 billion - or risk being fired in a shareholder mutiny led by activist investor Carl C. Icahn.

Spurred on by outraged shareholders, Icahn notified Yahoo yesterday that he will lead a revolt to oust Yang and the rest of the Internet company's board members unless they renew negotiations with Microsoft that fell apart May 3, when the two sides could not agree on a price.

To pressure Yahoo, Icahn has nominated an alternative slate of directors to replace the current board in an election at Yahoo's annual meeting, scheduled for July 3. If the uprising is successful, an Icahn-led board presumably would fire Yang as CEO and try to negotiate a sale to Microsoft.

Icahn disclosed that he has spent more than $1 billion snapping up 59 million Yahoo shares and options to give him a 4.3 percent stake in the Sunnyvale, Calif., company. He plans to seek approval from the Federal Trade Commission to acquire up to $2.5 billion in Yahoo stock, including his current holdings.

Icahn's challenge opens a new chapter in a saga that began Jan. 31, when Microsoft stunned Yahoo with a takeover bid that started out at $44.6 billion, or $31 a share, and then rose to $47.5 billion, or $33 a share, this month.

In a letter sent yesterday to Yahoo chairman Roy Bostock, Icahn lambasted the board's actions in rejecting Microsoft as "irresponsible" and "unconscionable," given that Yahoo's stock stood at $19.18 before Microsoft first made its bid. He urged the board to reopen the talks.

A Yahoo representative said the company would respond to Icahn "soon."

Yahoo shares rose 61 cents, or 2.25 percent, to finish yesterday at $27.75. That is the stock's highest closing price since Microsoft broke off talks.

While Icahn made it clear he wants Yahoo sold to Microsoft, there are no guarantees the software-maker is still interested in buying its rival.

A Microsoft spokesman declined to comment on Icahn's letter, saying the Redmond, Wash., company has "moved on."

If Yahoo cannot find a way to placate Icahn, the battle threatens to distract Yahoo and the rest of the company's management from their turnaround efforts, said James Post, a Boston University professor specializing in corporate governance and ethics.

"Senior management cannot concentrate on managing the business when they are concentrating on managing critical relationships with angry shareholders," Post said.

And there is no doubt Yahoo shareholders are furious, said Darren Chervitz, co-portfolio manager of the Jacob Internet Fund Inc., which owns about 100,000 Yahoo shares.

"There's a strong feeling that Yang and the board did not do their fiduciary duty," Chervitz said. "They had a very strong offer on the table and did everything to brush it aside, if not sabotage it."