A lawsuit has been filed against the makers of TV show "The Big Bang Theory" over the ownership of the song "Soft kitty".

The song has become the show's character Sheldon's comfort whenever he's sick or troubled. 

Late poet Edith Newlin wrote lyrics of the song "Warm kitty" in 1937. Her daughters, Margaret Perry and Ellen Chase, thought CBS should pay royalty, reports aceshowbiz.com.

They filed a lawsuit on Monday in a federal court in Manhattan, saying the "The 'Soft kitty' lyrics have played a prominent role in the development and portrayal of one of the central characters in the programme." 

Newlin's lyrics say: "Warm kitty/ soft kitty/ little ball of fur/ sleepy kitty/ happy kitty/ purr purr purr." 

The one used on the show only changes the order of the lyrics and the rest are the same. It was first sung in a 2008 episode by both Sheldon and Penny before it was repeated in seven more episodes. 

"The Big Bang Theory" made the song so popular that CBS made a merchandise out of it, including coffee mugs, hoodies, T-shirts, toys and key chains. 

The lawsuit said: "The 'Soft kitty' lyrics are among the best known and most popular aspects of 'The Big Bang Theory'. They have become a signature and emblematic feature of the show and a central part of the show's promotion.”