In a coda to one of the most notable music copyright lawsuits in years, Warner Music has agreed to pay $US14 million ($19.8 million) to settle claims over "Happy Birthday to You," after a judge ruled last year that the company's long-claimed copyright to the famous song was invalid.

Warner Music, through its publishing subsidiary Warner/Chappell, agreed after mediation to pay the settlement to a class of "thousands of people and entities" who had paid licensing fees to use the song since 1949, according to papers filed in federal court on Monday.

The terms of the settlement, which must be approved by the judge in the case, George H. King of United States District Court in Los Angeles, call for him to declare "Happy Birthday" part of the public domain.

The settlement would also grant $US4.6 million in fees to the lawyers for the plaintiffs, a group of independent artists and filmmakers who filed separate suits in 2013 that were later combined.

"Happy Birthday," which the suit calls "the world's most popular song," can be traced to another tune with the same melody but different lyrics: "Good Morning to All," written by Mildred Hill and her sister Patty, a kindergarten teacher in Kentucky, which was published in 1893. Variations with the familiar birthday lyrics began to appear in the early 20th century, but their authorship was uncertain. Warner/Chappell had controlled the song since 1988, when it acquired the song's last publisher.

"Happy Birthday to You” traces back to a song written in the 1800s by two sisters, Patty Smith Hill, left, and Mildred J. Hill. Photo: Adam Ferguson

The "Happy Birthday" case was closely followed in the music industry and among copyright specialists in part because of the song's popularity and the value of its royalties, which have been estimated at about $US2 million a year. The suit included developments like the last-minute discovery of a yellowed songbook from 1922, which the plaintiffs' lawyers called "a proverbial smoking gun" because it included a version of the popular birthday song without proper copyright notice.

After the judge's determination in September that Warner's copyright to "Happy Birthday" was invalid, the parties were set to go to trial, but they announced in December that they had reached an agreement. The terms of that agreement were confidential until the filing Monday.

The New York Times