SAN FRANCISCO – Dell's board rejected CEO Michael Dell's attempt to change the voting rules for his bid to buy the slumping personal computer maker, a decision that is likely to doom the deal.

But the endangered buyout could still get a reprieve if Michael Dell and his allies accept a counterproposal that would extend the voting period for a third time and allow a bigger pool of shareholders to cast ballots.

The rebuff announced Wednesday marks the latest blow that Michael Dell and his main backer, Silver Lake Partners, have absorbed since reaching an agreement with Dell Inc.'s board nearly six months ago to buy the Round Rock, Texas, company for $24.4 billion, or $13.65 per share.

A vote on Michael Dell's offer is scheduled to be held Friday. He had proposed a change designed to make it easier for that bid to win shareholder approval: In exchange for an extra $150 million, Michael Dell wanted to exclude shareholders who didn't vote from being counted as in the opposition column.

Dell's board turned down that demand but said it's willing to postpone Friday's vote and give shareholders several more weeks to consider the buyout proposal if Michael Dell and Silver Lake don't withdraw the slightly higher offer of $24.6 billion, or $13.75 per share, dangled before the company last week.

The original bid has faced stiff criticism from a vocal camp of critics led by two of Dell's largest shareholders, billionaire Carl Icahn and investment firm Southeastern Asset Management, who want to oust Michael Dell as CEO and replace the company's entire board to pursue different alternatives.