A group of filmmakers and a San Francisco musician are challenging Warner/Chappell Music’s claim to copyright ownership of what is perhaps the most popular song of all time — “Happy Birthday to You” — and on Monday a federal judge mapped out a plan of how the dispute will proceed.

At a hearing on Warner/Chappell’s effort to dismiss much of the case, U.S. District Judge George H. King instead indicated that he would seek to “bifurcate” the proceedings, first considering the question of whether the music publisher has ownership over “Happy Birthday” before going on to other legal claims, if needed, such as violation of California’s unfair competition laws and breach of contract. Still to be determined is whether the suit will be certified as a class action, as the plaintiffs claim that the publisher has collected millions over the years for a song it does not own.

The suits filed last summer contend that Warner/Chappell has been collecting license fees for the song even though it is in the public domain. The plaintiffs claim that the song “is neither copyrighted nor copyrightable,” and that the public began singing it “no later than the early 1900s.”

Attorneys for Warner/Chappell, however, say that they will submit as evidence certificates of a 1935 copyright registration for “Happy Birthday to You” that covers the lyrics and piano arrangements. One of the publisher’s attorneys, Kelly Klaus of Munger, Tolles & Olson, said at the hearing that they “think this case will end very quickly on the merits.”

The plaintiffs challenge that the song could have been copyrighted in 1935, arguing that the words were not original at the time of the registration, save for a second verse that is seldom if ever performed, said their attorney, Mark Rifkin of Wolf Haldenstein Adler Freeman & Herz. The plaintiffs also have contended that the scope of the 1935 copyright is limited to the piano arrangements.

Another question is whether two of the plaintiffs, Robert Siegel and Major Prods., will eventually be dismissed from the case as the statute of limitations for copyright claims is three years. In 2009, Siegel paid Warner/Chappell $3,000 to use “Happy Birthday” in his movie “Big Fan,” and that same year Majar paid the publisher $5,000 to use the song in the documentary film “No Subtitles Necessary: Laszlo & Vilmos.”

The other plaintiffs licensed the song more recently. Rupa Marya paid Warner/Chappell $455 to use the song in a live album, and the makers of a documentary on the history of the song, tentatively titled “Happy Birthday,” paid $1,500 to license the song.

The work has a long, convoluted history. The plaintiffs’ suit traces “Happy Birthday” to 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 sometime in the early part of the 20th century, but the sisters did not write those words. If there ever was a copyright to the song, the plaintiffs contend, it expired long ago, not later than 1921.

Warner/Chappell acquired the company that claimed ownership of the song, Birch Tree Hill, in 1998.