Billionaire investor Carl Icahn urged Dell Inc. shareholders to seek an appraisal of their stakes in a bid to block a proposed $24.4 billion buyout of the computer maker by founder Michael Dell.

Michael Dell and partner Silver Lake Management LLC didn’t obtain the right to opt out of the deal, which offers holders $13.65 a share, if a certain number of shareholders seek appraisal rights, Icahn said in an open letter to shareholders today.

“We believe the $13.65 merger price substantially undervalues your Dell shares, and we believe if you seek appraisal, you will receive more,” Icahn said in the letter.

Shareholders are set to vote on Michael Dell’s transaction on July 18. His proposal has the backing of Institutional Shareholder Services Inc. and two other shareholder-advisory firms. Icahn has proposed a stock buyback as an alternative to the CEO’s plan, which is opposed by about a fifth of Dell’s shareholders.

To succeed the buyout needs approval from a majority of investors excluding Michael Dell, who has about a 16 percent stake.