It is one of the most beloved and famous of all songs, belted out at countless gatherings for infants and octogenarians alike. Yet, Happy Birthday To You - far from being as free as a piece of cake at a party - is considered private property.

A federal lawsuit filed by a group of independent artists is trying to change that. In a recent filing, lawyers in the case said they had found evidence in the yellowed pages of an almost century-old songbook that shows the song's copyright - first issued in 1935 - is no longer valid.

A judge could rule on the case in the coming weeks. If the song becomes part of the public domain, it would cost the Warner Music Group, which holds the rights, millions of dollars in lost licensing fees.

It would also be a victory for those who see Happy Birthday To You as emblematic of the problems with copyright - a song that has long since outlived anyone involved in its creation, yet is still owned by a corporation that charges for its use.

"It is one of the few songs that you've heard for as long as you've lived, and you kind of think of it as a folk song," said Professor Robert Brauneis at the George Washington University Law School, who in 2010 published a sceptical study of the song's copyright.

The case also highlights the centrality of copyright claims to media businesses such as the music industry, for which the question of who owns the rights to a song could be worth millions. Advocates for rigorous copyright laws note that they protect musicians as well as the companies that represent them.

The industry was rattled in March, when a jury found that Robin Thicke's song, Blurred Lines, had copied Got To Give It Up, a 1977 hit by Marvin Gaye.

Part of the dispute over Happy Birthday derives from the song's byzantine publishing history.

Its familiar melody was first published in 1893 as Good Morning To All, written by Mildred Hill and her sister Patty. Birthday-themed variations began to appear in the early 1900s and, soon, Happy Birthday was a phenomenon, popping up in films and hundreds of thousands of singing telegrams in the 1930s.

Its appearance in a scene in an Irving Berlin show, As Thousands Cheer, in 1933 led to a lawsuit and, in 1935, the song's copyright was registered by the Clayton F. Summy Co, the Hill sisters' publisher.

The song changed hands over the years, and Warner acquired it in 1988 while buying its owner, Birchtree, as part of a publishing deal then reported to be worth US$25 million. According to some estimates, the song now generates about US$2 million (S$2.8 million) a year in licensing income, mostly from its use in TV and film.

Yet, even though it is widely performed at private gatherings, its copyright status leads to peculiar workarounds in public settings. Chain restaurants often come up with their own songs to avoid paying licensing fees, according to Prof Brauneis' paper. On live TV, it is not uncommon for an impromptu performance to be quickly silenced by producers.

Film-maker Jennifer Nelson, who is making a documentary about the song and first filed the lawsuit against Warner two years ago, said the company charged her US$1,500 to use it.

The case, which has been joined by other artists and is seeking class-action status, is being heard in federal court in Los Angeles. Plaintiffs want the song to be declared part of the public domain, and for Warner to return licensing fees dating back to at least 2009.

"Our clients want to give Happy Birthday To You back to the public, which is what Patty Hill wanted all along," said Mr Mark Rifkin, a lawyer for the plaintiffs.

Warner, which declined to comment for this article, contends in court filings that the copyright is valid. The song also generates hundreds of thousands of dollars each year for a non-profit group, the Association for Childhood Education International.

Happy Birthday To You has long been a prime target for critics of laws that regulate copyright.

Because of an extension made under the Sonny Bono Copyright Term Extension Act of 1998 - which was lobbied for heavily by Hollywood - the song will remain under protection until 2030.

"The fact that Happy Birthday To You is still under copyright is the most symbolic example of how copyright has expanded and overreached beyond its constitutional purpose," said Prof Kembrew McLeod at the University of Iowa, who has written about the song.

Prof Brauneis contended in his 2010 study that the song's copyright might not have been properly renewed when its initial term expired, in 1963. And lawyers for the plaintiffs in the Happy Birthday suit - for whom Prof Brauneis said he was working as an unpaid consultant - now say they have proof of deeper problems.

Late last month, they submitted evidence that they called "a proverbial smoking gun" - a 1922 songbook containing Good Morning And Birthday Song, with the birthday lyrics in the third verse.

Other songs in the book are given with copyright notices, but Good Morning And Birthday Song says only that it appears through "special permission" of the Summy Co.

Under the laws of the time, an authorised publication without proper copyright notice would result in forfeiture of the copyright, said lawyers involved in the case.

Further, under the 1998 law, anything published before 1923 was considered part of the public domain.

Warner argued that, even though earlier versions of the birthday song might have been published, they were not authorised by the sisters. Also, no copyright covered Happy Birthday, the label argues, until it was registered in 1935, so there was no copyright to be invalidated in 1922.

Both sides have asked for summary judgment, and Judge George King of the District Court in Los Angeles is expected to rule soon. He could deny both motions and hold a trial - raising the possibility of a strange proceeding in which all principal witnesses are long dead.

As part of the evidence submission last month, the plaintiffs included a paper trail showing how they tracked down the songbook.

It started with electronically scanned images from Warner of a 1927 edition of the same book, but with the publisher's crucial permission line about Good Morning And Birthday Song blurred. Lawyers for the plaintiffs searched for other copies of the book, and found one at the University of Pittsburgh; a 1922 edition was also located.

In a series of e-mail about the 1927 edition, a Pittsburgh librarian told Mr Rifkin that the songbook had been found at a university storage facility.

"Here you go," she wrote in sending it to him. "Surely the copyright hasn't lasted this long."

NEW YORK TIMES