Producers behind the TV series, which stars Kaley Cuoco and Jim Parsons, have been hit with a lawsuit from two women who claim lyrics from a poem written by their mother have been reproduced onscreen without proper permission.

They claim the show's song Soft Kitty is an altered version of a poem called Warm Kitty which was penned by their writer mother Edith Newlin around 1933 and published in a 1937 book of nursery rhymes.

The book was published by bosses at Willis Music who struck a deal with executives at Warner Entertainment in 2007 to allow the poem to be tweaked and used in The Big Bang Theory, but Ellen Newlin Chase and Margaret Chase Perry claim the deal was not valid because they own the copyright.

In the lawsuit, which was filed in New York City on Monday (28Dec15), they demand a cut of the profits from the companies behind the show, as well as unspecified damages, according to the New York Daily News.

The song Soft Kitty has been used in the show on numerous occasions, and the lyrics are also featured on The Big Bang Theory merchandise.