The proposed merger between CBS and Viacom has been withdrawn by Shari Redstone, vice chair of the board for both companies, sources told CNBC's David Faber.

Redstone, who had pushed for the companies to explore a merger, is reversing course and now backs acting CEO Bob Bakish and his plan to bring the Viacom back to prominence. Redstone now wants Viacom to remain an independent company, according to sources on both sides of the proposed deal.

The move was subsequently confirmed in a message Monday from National Amusements — the Redstone family's privately held company — to the boards of CBS and Viacom.

"Over the past few months, after careful assessment and meetings with the leadership of both companies, we have concluded that this is not the right time to merge the companies," the message said. "Following the management changes that the Viacom Board put in place, we have been very impressed with the forward-looking thinking and strategic plan being pursued under Bob Bakish's leadership."

The two companies had engaged in preliminary conversations about a merger, but CBS had yet to make a bid for Viacom.