Updated with Viacom response: Sumner Redstone’s National Amusements, which owns 80% of Viacom’s voting shares, just threw a wrench into CEO Philippe Dauman’s effort to sell 49% of Paramount to China’s Wanda Group.

The exhibition chain this morning says that it will oppose any deal he makes, alleging that it would “undermine rather than advance” shareholder interests —  for example by chilling “the interest of parties that may be interested in a larger transaction involving all of Viacom.”

“Paramount is one of Viacom’s most valuable assets,” NAI says. “Any short-term benefits that might result from a Paramount transaction would be outweighed by the severe negative impact on Viacom’s future strategic flexibility to best capitalize on this important asset. The complexities of joint-ownership of Paramount would undoubtedly extend far beyond Paramount into other divisions of Viacom and limit its options to take on partners.”

Viacom responds that it is “beyond understanding that ‘National Amusements’ would continue in its attempts to interfere with a potential transaction that would create a unique opportunity to drive long term value for both Paramount and Viacom, without even waiting for the facts. We will continue to pursue the best outcome for all of Viacom’s shareholders.”

Today’s statements follow reports that Dauman is honing in on an agreement with Wanda that could value all of Paramount at as much as $10 billion. That would be much higher than the value that shareholders ascribe to the studio.

Wall Street discounted the reports, given Redstone’s previous opposition to a Paramount sale — and the growing belief that Dauman won’t be able to hold on to his job. Viacom shares are down 1.4% this morning.

In addition to challenging the merits of a Paramount deal, NAI says that the decision shouldn’t be made by “individuals who may be leaving the board shortly.” The company wants to replace five of Viacom’s directors, including Dauman. Viacom says that Redstone, 93, is no longer competent to make such a decision. The matter is now before Delaware’s Chancery Court.

Assuming NAI prevails, its board picks “should not have their strategic options limited,” it says. “If the new board decides to pursue a sale of a minority stake in Paramount as part of a larger strategic plan for Viacom, we are confident that a buyer could then be identified and a sale consummated.”

As part of an agreement approved by the Delaware court, Viacom’s board must give NAI five days notice before making any decision that’s not considered ordinary business.

“The notice period is intended to provide National Amusements with the opportunity to oppose any such transaction,” it says. “If required, we will do so.”

NAI also wants to change the bylaws, requiring a unanimous vote for any deal involving Paramount. That would give Redstone and his daughter Shari, Viacom’s Vice Chair, veto power.