A group of booksellers, publishers and authors filed a lawsuit on Tuesday to stop a new law in Texas that would require stores to rate books based on sexual content, arguing the measure would violate their First Amendment rights and be all but impossible to implement.

The law, set to take effect in September, would force booksellers to evaluate and rate each title they sell to schools, as well as books they sold in the past. If they fail to comply, stores would be barred from doing business with schools.

“It will be a huge burden,” Valerie Koehler, the owner of Blue Willow Bookshop in Houston, said of the law. She estimates that schools account for some 20 percent of her store’s sales. “It’s unfathomable to think that we would need to rate every book, not only ones that we’d sell in the future to schools, but also any books we’ve sold in the past.”

The Texas law, and the legal battle to block it, reflects a new front in an ongoing culture war over book banning and what constitutes appropriate reading material for children.

In the past two years, book bans have surged in the United States, driven by conservative activists who have targeted books about race and racism or L.G.B.T.Q. issues and characters. An array of new laws have passed around the country, making it easier to remove books from libraries and placing new restrictions on the types of titles children can access.

While the fight has largely centered on books that are available in school classrooms and libraries, the legislation in Texas has drawn booksellers directly into the conflict.

Gov. Greg Abbott, who signed the bill into law in June, championed it as a way for parents to exercise more control over the books available to their children. “Some school libraries have books with sexually explicit and vulgar materials,” he said during the bill signing session. “I’m signing a law that gets that trash out of our schools.”

Many of the restrictions on books available in schools and libraries have been promoted under the banner of giving parents more choice over the content their children encounter. But the plaintiffs said that the Texas law would take decisions out of the hands of schools and parents and put the burden on vendors instead.

The law’s opponents also argue the legislation will increase the number of book bans in Texas, which already leads the country in removing books from schools, according to an analysis by the free speech organization PEN America.

The suit was brought by two Texas independent bookstores — Koehler’s Blue Willow Bookshop and BookPeople, in Austin — together with the American Booksellers Association, the Association of American Publishers, the Authors Guild and the Comic Book Legal Defense Fund. It was brought against state library and education officials who are responsible for implementing the law.

The complaint, which was filed in the U.S. District Court for the Western District of Texas, argues that the law violates the First Amendment because it requires booksellers to label books with subjective and potentially polarizing ratings, categorizing them as “sexually explicit,” “sexually relevant” or “no rating.” (Movie ratings, by contrast, are voluntary.)

Under the law, booksellers must submit a list of their ratings to the Texas Education Agency, which will list them on a website. If the state disagrees with the rating, it can overrule the bookseller and impose its own rating.

Schools would be prohibited from buying or providing books that are labeled “sexually explicit.” Books rated “sexually relevant” would be restricted, and can only be checked out by students with written parental consent.

“They’re trying to control what other people’s kids can read,” Cheryl L. Davis, general counsel of the Authors Guild, said of the legislators behind the measure.

Maria A. Pallante, the chief executive of the Association of American Publishers, said the law would force book vendors to act as “mouthpieces of the government” by “retaliating against them if they do not do the labeling.”

Under the First Amendment, the government cannot compel speech from private individuals or businesses.

This lawsuit is the latest attempt to push back on book removals. Other efforts include a suit filed in Arkansas, where a new law could send librarians and booksellers to prison if they fail to create a separate “adults only” area for material that might be “harmful” to minors. In Florida, a group of students and the authors of a children’s picture book recently sued a school district and the state’s board of education, saying their push to restrict access to books in school libraries was unconstitutional.

State library and education officials who are responsible for implementing the law did not immediately respond to requests for comment.

Even before it goes into effect, the Texas law has already had an impact. In Katy Independent School District, outside Houston, schools have stopped buying books and put all of their current titles in storage until they are rated, according to the complaint.

Charley Rejsek, the chief executive of BookPeople in Austin, said that complying with the law would be impossible. BookPeople — which takes its name from Ray Bradbury’s novel “Fahrenheit 451,” in which a group of people try to preserve books in a world where they are burned — was founded in 1970, Rejsek said. The store does not have records of titles sold over the last half century, much less a way to know which of them are still in circulation — but under the law, BookPeople would be responsible for rating those books.

“I don’t see a clear path forward for complying with the law as written,” she said. “I don’t know how I can rate them if I don’t have any records.”

Going forward, she said, BookPeople would have to read and rate many thousands of titles requested by school districts. Some might be in languages her staff cannot read, she said.

“I want to work with schools,” she said. “But I just literally can’t find a way to comply.”