You may not know it, but you could owe Warner/Chappell Music thousands of dollars from all the birthday parties you've attended in your life.

A federal lawsuit seeks to change that.

Warner/Chappell Music claims to own the copyright to the 120-year-old, 16-word song that is widely credited with being the best-known piece of music in the English language. That means anyone who performs the song publicly risks a $150,000 fine if they don't agree to pay a fee to the music group.

While the company doesn't actually come after private individuals for singing the song to their 3-year-olds, it technically could. And it does demand money anytime the song is sung on a television show or movie.

Good Morning To You Productions, which is a making a documentary film about the song, filed a lawsuit Thursday seeking to have the song returned to the public domain. It argues the copyright on the song expired in 1921, and that it should not have been forced to pay $1,500 for the rights to use the song in its documentary.

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Warner/Chappell Music is the music publishing arm of Warner Music Group. It claims on its website to own the copyright to more than 1 million songs.

According to the suit, the song was written by Mildred and Patty Hill and sold to Clayton Summy in 1893 for 10% of the retail sales of the sheet music. A company that Summy founded was eventually purchased by Warner Music Group in 1998, according to the suit. It argues that if Warner/Chappell owns any copyright, it's on a very limited piano arrangement published in 1935, not on the song itself.

And it is seeking class action status to have Warner/Chappell return all of the millions in fees it has collected. The suit argues that Warner/Chappell collects more than $2 million a year in copyright fees on "Happy Birthday."

Robert Brauneis, law professor at George Washington University and an expert on the copyright of "Happy Birthday," said his research suggests the copyright expired a bit more recently -- 1963. "But that's still a long time ago."

He said the reason Warner/Chappell has been able to continue to collect fees is that it hasn't been challenged.

Those who pay the most of the copyright fees -- movie studios, television networks or ad agencies -- don't want to go to war with rights holders over a relatively cheap song when Warner/Chappell could then raise the cost for using other songs for which it clearly owns copyrights.

"Until somebody mounts a challenge, you keep sending out collection notices," Brauneis said. "Even for a feature film, the $20,000 to $30,000 fee is not going to get too far when paying attorneys."

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People singing "Happy Birthday" to family members in their homes are specifically exempt from having to pay, Brauneis said. But he said those singing the song at a party in a public venue, such as a Chuckie Cheese, technically could be forced to pay a fee.

"A restaurant is a public performance. It doesn't matter whether somebody is getting paid," Brauneis said.

But fortunately, he said it's not worth it to Warner/Chappell to try to collect on those kind of "performances." So this writer will probably be safe tomorrow when leading a chorus of "Happy Birthday" to his daughter Katrina at the rock climbing facility where her 11th birthday party is being held.

Los Angeles-based Warner/Chappell could not be reached for comment Friday. The suit was filed in Federal Court in New York.