Shareholders with 378 million of around 1.5 billion total votes -- about 25.2 percent -- withheld support for Michael Dell's election to the company's board, the company said in a filing on Tuesday with the U.S. Securities and Exchange Commission (SEC).

The vote, held Thursday, is a sign of mounting investor frustration around a sagging share price and management practices that have included misleading financial reporting, observers and shareholders said.

Dell wields too much influence over the board and company as the CEO and chairman, said Brandon Rees, deputy director of AFL-CIO's office of investment, which holds Dell stock. A letter sent by the AFL-CIO and American Federation of State, County, and Municipal Employees (AFSCME) on Aug. 3 encouraged shareholders to withdraw their support for Dell as chairman.

A Dell spokesman, David Frink, said in an e-mail that Michael Dell has the support of its shareholders and board.

"The Dell Board of Directors recently expressed its unanimous confidence in Michael Dell's leadership of the company. A majority of shareholders agreed and last week reelected Mr. Dell to the board," Frink said.