NEW YORK (Reuters) - As she seeks to combine CBS Corp (CBS.N) and Viacom Inc (VIAB.O) into a single company, media mogul Shari Redstone risks losing her most talented executive, whose leadership could be critical to ensuring the proposed merger succeeds.

Shari Redstone arrives for Variety's Power of Women luncheon in New York City, U.S., April 21, 2017. REUTERS/Brendan McDermid

Such is the difficulty facing Redstone. Her longtime friend and business ally, powerful CBS Chief Executive Les Moonves, has emerged as one of the biggest obstacles to a potential CBS-Viacom tie-up, according to people familiar with the matter. Concerns are that his objections could either doom the transaction or that he might not lead the new entity, the people said.

Moonves is chafing at Redstone’s insistence on installing Viacom’s CEO as his second-in-command and likely successor of a merged company, the people say. CBS and Viacom announced in February that they were exploring a potential deal and in recent weeks have begun negotiations.

At stake is the future of Redstone’s media empire, which is facing pressure to scale up at a time when competitors are gobbling rivals to stay relevant. Her family’s company, National Amusements Inc, controls both CBS and Viacom. Redstone favors merging the two to gain size and leverage in negotiating better programming prices from traditional cable and satellite customers, and to better compete with emerging players such as Netflix, the people said.

But she may have to decide if she is willing to do it without her top captain to steer the ship.

Redstone wants Moonves to run the combined company, according to the people familiar with the situation, all of whom wished to remain anonymous because discussions are confidential. But she is contemplating moving forward without him

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