Los Angeles ? Walt Disney Co. chief executive Michael Eisner unexpectedly offered his resignation from the entertainment company he has led for two decades, but he will not be leaving the executive suite anytime soon.

In a letter to Disney’s board dated Thursday, Eisner said he intended to step down on Sept. 30, 2006, when his contract expires.

“Until then I shall continue to exert every effort to help the company achieve our goals, to assist the board in selecting the new chief executive officer, and to make the transition expeditious, efficient, and smooth and easy,” Eisner wrote.

Eisner — known as a workaholic who reads scripts at night, watches every television pilot and even helps in the design of theme parks — helped transform Disney into a media powerhouse but recently fought off a shareholder attempt to oust him.

Disney has fallen on hard times in recent years amid sinking ratings at ABC Networks and some questionable acquisitions. Disney stock has stumbled, and investors have begun to question Eisner’s control over his board of directors. Eisner did not say whether he would seek to retain his seat on the board.

Critics called for Eisner to step down sooner and resign his seat on the board as well.

“It is not clear to us how a two-year lame duck CEO will benefit shareowners, and his continued presence on the board would prevent the company from the clean break that is needed to restore investor confidence,” said Sean Hannigan, president of the California Public Employees Retirement System, the nation’s largest pension fund and a major investor.

The timing of the announcement surprised investors and many of the company’s top executives. It set off a guessing game about possible successors and raised questions about whether the move would silence his most vocal critics.

“We respect Michael’s decision,” Disney’s chairman, former Sen. George Mitchell, said in a prepared statement, speaking for the full board. “We are deeply grateful for his remarkable 20 years of creative leadership.”

Eisner has strongly endorsed Disney president Robert Iger as a potential successor.

Eisner joined the company in 1984 along with former Warner Bros. executive Frank Wells at the behest of Roy E. Disney, nephew of company co-founder Walt Disney.

Eisner and Wells transformed Disney by revitalizing its dormant animation efforts, expanding the theme parks and by purchasing Capital Cities/ABC in 1995, thus adding ABC, ESPN and other networks to the mix.

But soon after the ABC deal, the company began to struggle. Last year, Roy Disney and ally Stanley Gold, once Eisner’s strongest backers, resigned from the board and championed an effort to oust him.

Disney’s stock rose on the news Friday, trading up 30 cents at $23.16.