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In a court battle involving perhaps the only song more popular than “Blurred Lines,” a federal judge is set to decide whether a Los Angeles-based music publisher has unlawfully been collecting licensing fees for the copyright to “Happy Birthday to You.”

U.S. District Judge George King of the Central District of California heard more than two hours of arguments on Monday on whether to declare Warner/Chappell Music Inc.’s copyright invalid and find that “Happy Birthday to You” should be in the public domain.

At stake are potentially millions of dollars in licensing fees to what the complaint calls the “world’s most popular song.”

The case was brought in 2013 by two New York music producers, a California musician and a film producer who each paid between $455 and $3,000 in licensing fees to Warner/Chappell Music, the music-publishing arm of Warner Music Group Corp., to perform the song.

The class action, filed on behalf of anyone who was forced to pay similar fees starting on June 18, 2009, sought a declaratory relief and the return of “millions of dollars of unlawful licensing fees.”

U.S. District Judge George King of the Central District of California bifurcated the case so that the declaratory-judgment action would proceed ahead of other claims, including those for licensing fees and damages under California’s unfair-business-practices and false-advertising laws.

King heard arguments on summary-judgment motions on Monday. When he will rule on those motions is not certain.

It’s not the first time the song’s copyright has been litigated, with most of the lawsuits filed in the 1930s and 1940s. But none resolved the issue of who owns the copyright to the ubiquitous version of “Happy Birthday to You.”

“Nobody’s ever decided who owns the copyright to ‘Happy Birthday to You,’ ” said Mark Rifkin, a partner at New York’s Wolf Haldenstein Adler Freeman & Herz, who represents the plaintiffs in the case.

Warner Music Group spokesman James Steven and its attorney, Kelly Klaus, a partner at Los Angeles-based Munger, Tolles & Olson, declined to comment.

The song’s copyright has engendered debate in recent years, particularly after Robert Brauneis, co-director of the Intellectual Property Law Program at George Washington University Law School, cast doubt in a 2009 law review article that “Happy Birthday to You” was copyrightable.

In the current case, both sides agree that sisters Mildred and Patty Hill composed and wrote the melody to the song “Happy Birthday to You”—then called “Good Morning to All”—in 1889 or 1890. The sisters later sold the copyright to Clayton Summy, who published a songbook called “Song Stories for the Kindergarten.”

But they disagree on most everything else about the song’s history, according to a Nov. 25 court document outlining their opposing motions for summary judgment.

Warner/Chappell is relying on a 1935 copyright obtained by Summy’s company, Clayton F. Summy Co., which it claims included the now familiar “text,” or lyrics, to “Happy Birthday to You.” Warner/Chappell acquired Summy’s successor, Birch Tree Ltd., in 1988.

Plaintiffs allege those copyrights were for piano arrangements and that “Happy Birthday to You” by then had reached widespread popularity, putting it in the public domain.

“Our argument has been all along that the copyright in 1935 only covered that particular piano arrangement,” Rifkin said. “There’s no evidence they ever acquired rights to the song ‘Happy Birthday to You’ from anyone before 1935 when they registered the copyright.”

Brauneis, who has consulted with the plaintiffs but is not a paid consultant, agreed. “In my view, that was not a mistake, but occurred because everyone thought at the time that the song was old, and that the only thing that was new were the arrangements,” he wrote in an email. “Later, it became convenient to construct a theory under which the basic song first gained copyright in 1935, even though we know that it had been composed and was well known before 1900.”