CEO Steve Jobs at an Apple event last October to introduce Mac OS X Lion.

An influential investor advisory group has added its support to an Apple shareholder proposal that calls on the company to disclose a succession plan for CEO Steve Jobs.

Jobs announced last month that he was taking a indefinite medical leave from company--his third in recent years--and handing over day-to-day responsibility to Chief Operating Officer Tim Cook, who filled in for Jobs while he was on medical leaves in 2004 and 2009. However, concern over Jobs' current health condition and shareholder complaints about a lack of information from the company regarding his condition have led to demands that Apple reveal an executive succession plan.

Institutional Shareholder Services has endorsed a proposal for such a plan offered by the Central Laborers' Pension Fund, an Apple investor that holds about 11,500 shares, or about a thousandth of 1 percent of outstanding shares. The proposal calls for Apple's board of directors to report each year on the state of its succession plan.

In ISS' analysis, which was announced today by the Laborers' International Union of North America--another supporter of a public succession plan, the proxy voting service said:

Apple shareholders will likely get a chance to vote on the proposal at the next shareholder meeting on February 23, the labor union said in a statement today.

Apple representatives did not immediately respond to a request for comment, but Apple's board of directors said in regulatory filings in January that it has recommended shareholders vote against the proposal. Apple said it already has established a succession plan and disclosing it publicly would only hurt the company's ability to retain and recruit top executive talent.

In January 2009, Jobs said that he was suffering from a hormone imbalance that was impeding his body's ability to absorb certain proteins. In April of that year, Jobs underwent liver transplant surgery and returned to work by early July. In August 2004, Jobs underwent successful surgery to treat a rare form of pancreatic cancer, which sidelined him until September of that year.

While Cook filled in as Apple's chief executive on both occasions and demonstrated an ability to execute on Jobs' big-picture plans already in place, what makes investors nervous is the thought of an Apple without Jobs--its founder, visionary, and public face.

"There's a lot of indication that Apple is set up for the future," DisplaySearch analyst Richard Shim previously told CNET's Erica Ogg. "They have a lot of talented personalities. The problem is that all these talented people report to one of the most charismatic and influential guys in technology."

Besides Cook, that group includes Phil Schiller, the head of marketing, who hones Apple's sales pitch' CFO Peter Oppenheimer who manages Apple's bottom line' Jobs' design guru, Jonathan Ive' the senior vice president of Apple's vast retail operation, Ron Johnson' and the men who head up hardware engineering (Bob Mansfield) and software engineering, (Bertrand Serlet).