It is the most recognised song in the world, according to the Guinness World Records.

But for decades, instead of launching into a rousing rendition of Happy Birthday To You, characters in films and TV shows have instead sung a strange facsimile to mark a celebration.

The reason is that Happy Birthday, sung at millions of parties around the world every day, is copyrighted, and those using it for 'commercial ends' are obliged to pay owners Warner Music for the privilege.

Now, through her film company, documentary maker Jennifer Nelson is launching a lawsuit against the music giant.

Nelson claims that the song has in fact been in the public domain for years, and that Warner holds only a narrow copyright for a certain arrangement of Happy Birthday.

She discovered who the purported owners of the the song were after setting out to make a film about its history, and was informed that she would have to pay Warner $1,500 (£995) to use it.

The song's melody was written in 1893 by kindergarten teachers Patty and Mildred Hill as a ditty to be sung by children at the start of the day, and originally had the words Good Morning to All.

By the early 1920s the original copyright had expired, and versions with the lyrics Happy Birthday to You had appeared. It was not until 1935 that Jennifer Hill, Patty and Mildred's sister, copyrighted the song with the Clayton F Summy Company, before Warner bought the Summy back catalogue in 1988.

But Nelson claims that this copyright was only for the original melodic arrangement of the tune.

The lawsuit states: "Irrefutable documentary evidence, some dating back to 1893, shows that the copyright to 'Happy Birthday to You' ... expired no later than 1921 and that if defendant Warner/Chappell owns any rights to 'Happy Birthday to You', those rights are limited to the extremely narrow right to reproduce and distribute specific piano arrangements for the song published in 1935."

"Significantly, no court has ever adjudicated the validity or scope of the defendant's claimed interest in Happy Birthday to You, nor in the song's melody or lyrics, which are themselves independent works."

Nelson is demanding that Warner return the estimated $50 million it has earned in royalties from the song.

"Before I began my film-making career, I never thought the song was owned by anyone," Nelson told the New York Times. "I thought it belonged to everyone."