Sumner Redstone and his daughter Shari Redstone have pulled their support for a merger of CBS Corp. and Viacom Inc., a big bet that struggling Viacom can turn around its fortunes amid a fast-changing media landscape.

In a statement Monday, the Redstone family's holding company National Amusements—which has a majority voting stake in both firms—said that after reviewing a tie-up "we have concluded that this is not the right time to merge the companies."

A marriage would've brought the two companies together a decade after they split, and the news comes after a year of corporate drama at Viacom during which Mr. Redstone's daughter, Shari, has won great sway as president of National Amusements.

In late September, National Amusements said it believed a merger could "allow the combined company to respond even more aggressively and effectively to the challenges of the changing entertainment and media landscape."

Shares of Viacom fell 6.2% in early trading, while CBS fell 2.5%.

A combination would have brought under one roof CBS, a prime-time ratings juggernaut, and Viacom's struggling networks, such as MTV and Comedy Central, which typically pursue younger audiences that have been drawn to streaming services such as Netflix. Viacom also has been struggling with its Paramount Pictures studio.

In the statement Monday, National Amusements called for a focus on the companies' "independent paths forward."

The Redstone firm said it is encouraged that the companies can thrive separately, citing new Viacom Acting Chief Executive Bob Bakish's "forward-looking thinking and strategic plan" and confidence that Les Moonves, chief of CBS, could continue to deliver strong results there.

Remaining separate companies puts more pressure on Viacom to perform than CBS, which CBS has performed better in recent years thanks to the strength of its broadcast network and Showtime cable channel amid a growth in fees from pay-TV distributors. It is also in the process of shedding its CBS Radio unit. Meanwhile, Viacom has remained plagued by continued soft performance at its networks and a weaker showing at the box office.

Write to Joshua Jamerson at joshua.jamerson@wsj.com

(END) Dow Jones Newswires

December 12, 2016 10:25 ET (15:25 GMT)

Copyright (c) 2016 Dow Jones & Company, Inc.
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