Dell Inc. investors should accept founder Michael Dell’s $24.4 billion leveraged buyout plan for the company, Institutional Shareholder Services Inc. said, in a significant endorsement of the controversial deal.

ISS, the biggest shareholder-advisory firm, cited a 25.5 percent premium to Dell’s unaffected share price, the certainty of value provided by an all-cash offer and the transfer of risk given Dell’s deteriorating PC business among reasons for supporting the bid. The chief executive officer proposed to repurchase Dell shares at $13.65 each with partner Silver Lake Management LLC.

ISS’s influential recommendation could sway the final outcome because institutional investors look to ISS’s findings for guidance on how to vote their shares. It also lessens pressure on Dell, who is contributing his 16 percent ownership in the company at $13.36 apiece compared to the $13.65 offered to other shareholders and another $750 million in cash, to sweeten his offer.

Dell and Silver Lake Management aren’t planning to sweeten their buyout offer because the proposal they made in February represents a fair and significant premium to where the stock would trade if the deal fell apart, people with knowledge of the situation said July 5.

The ISS recommendation is the latest development in a five-month tug of war over Dell, which is now headed into its final act. Dell shareholders are scheduled to vote July 18 on the leveraged buyout proposal at a meeting at the company’s Round Rock, Texas, headquarters. CEO Dell has said taking the company private will let him rebuild it as a supplier of data-center equipment and software to reduce reliance on the flagging personal-computer market after years of lackluster growth.