You may have noticed something very strange about birthdays in movies and on TV. Namely, that the lucky recipient of the presents rarely has Happy Birthday sung to them - instead they might get For He’s a Jolly Good Fellow, a song only heard in movies and on TV. That’s because Happy Birthday - often claimed to be the world’s most popular song - has been under copyright to the publisher Warner Chappell, which has zealously enforced its right to royalties and earns an estimated $2m a year from Happy Birthday.

Now, however, it seems as though Warner Chappell might have made a mistake. Two years ago, film-makers working on a documentary about the song filed a lawsuit claiming the song should not be under copyright, after being told they would have to pay $1,500 to use it in their film. Director Jennifer Nelson has been seeking the return of that money and is also representing a class of plaintiffs who have also been charged for using the song.

Now attorneys for Nelson’s Good Morning to You Productions have found a songbook from 1927 containing Happy Birthday, with no copyright notice - predating Warner Chappell’s copyright by eight years. The songbook was in documents handed over by the publisher this month, which were “mistakenly” not produced during the discovery period in the case, which ended more than year ago.