Mr. Redstone often said he would live forever and never publicly unveiled detailed succession plans for the two public companies. Rather than clarify the situation, the rare statements he made about the future leadership of the two companies often fueled more speculation and uncertainty, setting the stage for what many analysts expect to be a fierce and litigious struggle for power.

Most recently, after new reports in May stated that Ms. Redstone was in line to take over for her father as chairman of CBS and Viacom, Mr. Redstone released a statement through a lawyer that decisions about who would succeed him would be made “by the boards of the respective companies and not by any individual.”

“Despite press reports to the contrary, such decisions have not yet been made,” Mr. Redstone’s statement said.

The executives who prevail will set the agenda at two of the world’s largest entertainment companies as they try to position themselves for a digital future. Media bankers and analysts also are preparing for CBS and Viacom to be involved in a new a wave of deal activity that could reshape the media landscape in the months and years to come.

“Thinking about what this change for CBS and Viacom could mean for the broader media landscape, we believe that changes for the Redstone-controlled empires could be the trigger for media company consolidation as each company will be forced to figure out if they are a buyer or seller in the coming months,” Michael Nathanson, an analyst with MoffettNathanson, said in a research note.

While some media bankers have speculated that Mr. Redstone’s death would make way for Viacom and CBS to reunite, Mr. Moonves is said to not support that idea, according to people briefed on his thinking.