CBS Corp. moved to break free from the Redstone family’s grip and thwart what it fears would be a forced merger with Viacom Inc., escalating a yearslong power struggle over the fate of the two media giants.

CBS filed a lawsuit Monday against the Redstones and their family holding company and invoked a little-known provision in the CBS corporate charter that it claims would allow it to issue voting shares to all stockholders, significantly diluting the voting power that the Redstones have held over CBS for nearly two decades.

The maneuver amounts to the triggering of what has been described as a “nuclear option” in the long-running power struggle between Shari Redstone and CBS Chief Executive Leslie Moonves, who has resisted her efforts to merge CBS and Viacom for two years.

In the lawsuit filed in a Delaware court, CBS and a special committee of independent directors sued Ms. Redstone, her father, Sumner Redstone, and their National Amusements Inc. holding company. CBS and the committee say they are seeking to prevent the Redstones and their company from breaching their fiduciary duties and causing irreparable harm by forcing a merger of CBS and Viacom, among other issues.

National Amusements controls almost 80% of the voting stock in both companies, giving it power to decide shareholder votes and elect directors.