The development puts in jeopardy Mr. Dauman’s position as chief executive of Viacom and is viewed as a major coup by Shari E. Redstone, Mr. Redstone’s daughter, who is the vice chairwoman of Viacom and CBS as well as a member of the trust. Long ostracized by her father, Ms. Redstone has said recently that the two have patched up their relationship.

Ms. Redstone does not to see eye-to-eye with Mr. Dauman or approve of his stewardship of Viacom, which has suffered a 40 percent plunge in the stock price in the last year. In February, Ms. Redstone voted against Mr. Dauman’s ascent to the chairman position of Viacom.

Shortly after news of Mr. Redstone’s decision emerged — first reported by Fortune — Mr. Dauman issued a statement of denunciation.

“They are a shameful effort by Shari Redstone to seize control by unlawfully using her ailing father Sumner Redstone’s name and signature,” Lex Suvanto, a spokesman for Mr. Dauman, said in a statement.

“As she knows, and as court proceedings and other facts have demonstrated, Sumner Redstone now lacks the capacity to have taken these steps,” Mr. Suvanto said. “Sumner Redstone would never have summarily dismissed Philippe Dauman and George Abrams, his trusted friends and advisers for decades.”

In the statement on behalf of Mr. Redstone, Mr. Tu said: “The public statement made on Mr. Dauman’s behalf attacking Mr. Redstone’s capacity is disappointing and incorrect, as reflected by Mr. Dauman’s own testimony given under oath less than six months ago that Mr. Redstone is as ‘engaged, attentive and as opinionated as ever.’ That is exactly the Sumner Redstone who made these decisions today.”

Ms. Redstone could not be reached for comment.

The development comes a little more than a week after a trial over his mental competency was dismissed in a California court. The judge overseeing the case sided with Mr. Redstone, but made clear that he had not made a decision about whether or not Mr. Redstone was mentally competent. The judge said that a videotaped testimony of the media mogul showed that it was “not in dispute that Redstone suffers from either mild or moderate dementia.”