They had cast the actors. Sold the tickets. Rehearsed the scenes.

But now, across America, small theaters are canceling productions of Harper Lee’s “To Kill a Mockingbird,” citing a threat of litigation from a powerful, sharp-elbowed Broadway producer related to a contract that dates back half a century.

The theaters were planning to stage an adaptation of the novel by the playwright Christopher Sergel, which has been widely staged by adults and students for decades. Lawyers for the producer Scott Rudin, backed by the Lee estate, are telling the theaters that their productions are no longer permissible because there is a new adaptation, by the screenwriter Aaron Sorkin, which opened on Broadway in December.

Mr. Rudin is the lead producer of the new adaptation. In January, he asserted what he called his exclusive stage rights in forcing the shutdown of a British touring production of the Sergel version.

Now he is making the same claim in the United States, leaving small theater companies scrambling and creating financial shortfalls for several tightly budgeted nonprofits. The Kavinoky Theater in Buffalo was two weeks away from staging “Mockingbird” when it received a cease-and-desist notice from Mr. Rudin’s lawyer.