The world's most popular English-language song is potentially free from copyright after a federal judge ruled on Tuesday that filmmakers challenging Warner/Chappell Music's hold on "Happy Birthday to You" should be granted summary judgment.

According to the opinion on Tuesday from U.S. District Judge George H. King, "Because Summy Co. never acquired the rights to the Happy Birthday lyrics, Defendants, as Summy Co.’s purported successors-in-interest, do not own a valid copyright in the Happy Birthday lyrics."

The ruling means that Warner/Chappell will lose out on $2 million a year in reported revenue on the song. Unless something happens at an appellate court or unless someone else comes forward with a valid claim of ownership to the song, filmmakers like director Jennifer Nelson — who sued in 2013 over demands as much as six figures to license — will no longer have to pay to feature "Happy Birthday" in motion pictures and television shows.

The "Happy Birthday" song dates to late 19th century work by a schoolteacher named Patty Smith Hill and her sister Mildred Hill.

The Hill sisters later assigned rights to a publishing company owned by Clayton Summy. Later, copyright registrations were made by Sumny's company on "Happy Birthday." Warner/Chappell has been contending that the 1935 registration covered both the piano arrangement as well as nearly universally known lyrics. The melody wasn't in contention. Both sides agree it is now in the public domain.

Today's opinion (read here) rejects Warner's argument that a copyright entitles them to a presumption of validity with the judge noting that it isn't particularly clear whether the registration included the lyrics. Furthermore, the ruling establishes that rights never properly transferred.

"Defendants ask us to find that the Hill sisters eventually gave Summy Co. the rights in the lyrics to exploit and protect, but this assertion has no support in the record. The Hill sisters gave Summy Co. the rights to the melody, and the rights to piano arrangements based on the melody, but never any rights to the lyrics."

With a nod towards plaintiffs' early contentions in the case that the song appeared in early 20th century schoolbooks, the judge adds that the Hill sisters never objected to publication until 1934 — four decades after they wrote the song. Even then, the Hill sisters asserted rights on the melody, not the lyrics.

The judge rules that plaintiffs can't score on a claim of copyright abandonment — it would have been a triable issue — but there's still enough in the record to give them victory on another aspect. Even if the Hill sisters still held common law rights to the lyrics when they made their later deals with Summy, the judge says there's no plausible evidence to support the theory they gave those rights to Summy Co.

This dispute is hardly over. Among other things, the plaintiffs represented by attorneys including Randall Newman and Mark Rifkin are contending that Warner should have to return millions of dollars in licensing fees. The issue of class certification and damages will come later.

"We are looking at the court's lengthy opinion and considering our options," said a Warner/Chappell spokesperson.