A sale of any part of Paramount Pictures would now require the approval of the entire Viacom board — including Sumner Redstone.
National Amusements, the majority shareholder of Viacom, announced Monday that it would change its bylaws specifically to require any transaction affecting Paramount Pictures to be unanimously approved by the Viacom board of directors.
The new bylaws will make it more difficult for Viacom to sell a minority stake in Paramount, as Viacom CEO Philippe Dauman had said he was in aiming to do.
A Viacom spokesman told Reuters the new bylaws are "illegitimate actions" that are "at odds with good corporate governance."
Viacom's board includes chairman emeritus Sumner Redstone and his daughter Shari Redstone. Sumner Redstone in May expressed opposition to selling Paramount.
It comes as Sumner Redstone and Dauman have been embroiled in a lawsuit over Redstone's mental competence. The company said the action was not related to the legal case, but rather, "to protect the long-term interest of Viacom's stockholders."
"While National Amusements is not opposed to a transaction that would unlock value at Paramount, it firmly believes that any proposed transaction should be thoroughly vetted and approved by Viacom's full Board," National Amusements said in a statement.
— Reporting by CNBC's David Faber.