Sumner Redstone's National Amusements Inc. is preparing to call on CBS Corp. and Viacom Inc. to explore a merger, two people familiar with the matter said on Wednesday.

NAI is the controlling shareholder of both media companies, which separated from each other 10 years ago.

A merger would give Viacom, whose networks include Comedy Central, Nickelodeon and MTV, a seasoned media executive at the helm with CBS CEO Leslie Moonves at a time when its TV ratings and ad revenue have been in decline, many investors believe.

The possible benefits for CBS are less clear, however, beyond the company's gaining scale, which could help with negotiations with cable and satellite distributors. CBS's ratings and ad revenue have been strong over the past few years and Moonves has resisted the idea of a recombination, other people familiar with the situation have said.

Viacom shares jumped as much as 7 percent after the Reuters story was published, and were trading up 1.7 percent at $36.07 in New York in early afternoon, giving the company a market value of $14.3 billion. CBS shares jumped as much as 4 percent and were hovering around $52.83, up 1.5 percent, giving the company a market capitalization of $23.2 billion.

NAI may contact the two companies as early as this week to ask them to form independent board committees to explore a combination, one of the sources said.

The sources asked not to be identified because the deliberations are confidential. Representatives for NAI, Viacom and CBS declined to comment.

National Amusements, the privately held movie theater company owned by Redstone and his daughter Shari Redstone, own 80 percent of the voting shares of Viacom and CBS.

Viacom was spun off from CBS in 2006, but Shari Redstone has been in favor of recombining the two under the leadership of Moonves, sources have previously told Reuters.

Industry speculation that the two might tie up has increased over the past few weeks, after the Redstones prevailed over a power struggle that resulted in the departure of Viacom Chief Executive Officer Philippe Dauman.

However, Moonves has indicated he is not warm to the idea. "We are never going to do something that is bad for CBS shareholders," he told investors at Bank of America Merrill Lynch's Media, Communications and Entertainment Conference earlier this month, adding that the two companies were not "in active discussions."

Last week, Viacom, which has more than $12 billion in debt, slashed its dividend by half to 20 cents per share and said it expects a fourth-quarter adjusted profit of 65-70 cents per share. Analysts on average were expecting 89 cents per share, according to Thomson Reuters I/B/E/S.

Viacom, which also owns Paramount Pictures, announced last week that its interim CEO, longtime Viacom exec Tom Dooley, would be leaving in November, leaving a hole at the top of the company as its board navigates strategy going forward.