It may come as a surprise to many that “Happy Birthday to You,” perhaps the most popular song in the English language, falls under copyright, or at least Warner/Chappell Music claims that they own it.

The makers of a documentary on the song, dating to 1893, are challenging the publisher’s collection of royalties for uses of “Happy Birthday,” claiming that the if there ever was a valid copyright to the work, it expired no later than 1921.

In a suit filed on Thursday, Good Morning to You Prods. is asking a New York federal court to issue a declaratory judgment that the work is in the public domain. It also is asking that a class be certified for the return of licensing fees that Warner/Chappell has collected over the years.The suit claims that the publisher, a unit of Warner Music Group, collects at least $2 million a year in license fees.

A spokesman for WMG did not immediately return an inquiry for comment.

The idea that “Happy Birthday” falls under copyright has been the subject of previous dispute, and even justices of the Supreme Court have referred to the song’s claimed legal protections in a landmark case over a 1998 copyright extension act.

The production company says that when it asked for a synchronization license from Warner/Chappell last year, the publisher charged $1,500. The company paid it, fearing that otherwise it would face a legal penalty.

The suit says that “Happy Birthday” originated in an 1893 manuscript for sheet music that included the song, “Good Morning to All,” which was written by Mildred J. Hill and her sister, Patty Smith Hill. The lyrics to “Happy Birthday” were adapted to the melody of that song some time in the early part of the 20th century, but the sisters did not write those words. That is where it gets murky.

The work has a long, convoluted history, with Warner/Chappell acquiring the company that claimed ownership of the song, Birch Tree Hill, in 1998.

The suit does acknowledge that if Warner/Chappell owns any rights to the song, it is an “extremely narrow right to reproduce specific piano arrangements” published in 1935.