NEW YORK - Michael Dell and his investor allies modestly boosted their buyout offer Wednesday for the struggling computer giant he founded and asked the board to change the method for shareholder approval.

The move came amid a hotly contested battle over the bid to take Dell (IW 500/24) private, a plan opposed by key shareholders as undervaluing the former No. 1 computer maker.

The new offer from Michael Dell and the private equity firm Silver Lake Partners was increased to $13.75 per share, from $13.65 a share, on condition of a change in the shareholder vote process.

The proposal seeks an approval process that would require a majority of shares voted, instead of a majority of all outstanding shares not affiliated with Michael Dell.

A statement from Michael Dell and Silver Lake said an estimated 27% of unaffiliated shareholders had not yet voted, suggesting that the plan could fail under the previous procedures.

"The presumption that these shares should be treated as if they had voted against the transaction is patently unfair," the statement said.

The new bid adds some $150 million to the $24.4 billion offer, according to a letter to shareholders.

A shareholder meeting set for Wednesday after a postponement last week was rescheduled for Aug. 2, the company said.

"Our Best and Final"

The letter said the new offer was "our best and final proposal" and set a deadline of 2200 GMT Wednesday for the board to endorse the proposal and new voting procedure. The deadline later was extended to August 2.

"We are not willing to discuss any further increase in the merger consideration, nor are we willing to increase the merger consideration to $13.75 per share without the change to the Unaffiliated Stockholder Approval requirement," the letter said.

"We believe that $13.75 per share is a full and fair price... The will of the majority of the unaffiliated shares voting on the transaction should not be thwarted by an unfair standard that counts unaffiliated shares not voting as 'no' votes."

The move came amid stiff opposition to the buyout by some shareholders who argued that Michael Dell's bid undervalued the former No. 1 computer maker.

Opponents have been galvanized by corporate raider Carl Icahn, with some other institutional investors also opposing the effort. The Wall Street Journal last week estimated that holders of at least 30% of Dell shares opposed the plan.

Michael Dell and his affiliates hold some 15.6% of Dell shares but cannot participate in the shareholder vote. Icahn and an investor group that has backed him, Southeastern Asset Management, together hold some 13%.