Image copyright Getty Images Image caption The show is also aired in the UK on Channel 4 and E4

The daughters of a teacher who wrote a poem about a "soft kitty" are suing CBS, the network behind The Big Bang Theory, for copyright violation.

According to the case filed by Ellen Newlin Chase and Margaret Chase Perry, the show used their mother Ellen's lyrics without buying the rights.

The show's characters have periodically sung a lullaby including the lyrics "soft kitty, warm kitty".

Mrs Newlin's daughters have also sued other media companies over the issue.

Calls to CBS and other companies named in the case were not immediately returned on Monday.

Mrs Newlin died in 2004, having worked as a nursery school teacher in Alstead, New Hampshire, for about 35 years. Her daughters still live in the small town.

The legal case claims that the "soft kitty" lyrics written by Mrs Newlin in the 1930s have been used in their entirety on at least eight episodes of the show since March 2008.

Blog post

It says the lyrics have also been used in merchandising, including on T-shirts, mouse pads, mobile phone covers, wallets, air fresheners, fridge magnets, toys and other products.

"The Soft Kitty lyrics are among the best-known and most popular aspects of `The Big Bang Theory," the claim states.

"They have become a signature and emblematic feature of the show and a central part of the show's promotion."

According to the claim, Ellen Newlin Chase discovered The Big Bang Theory's use of the lyrics in August 2014, when she was researching her mother's history for an article she was writing and came across a blog post discussing the use of the lyrics on the show.

The lawsuit says that in 2007, Warner Bros. Entertainment and the show's other producers decided they wanted to use the lyrics and sought permission from Willis Music Co., a Kentucky-based company that had published them in a book called Songs for the Nursery School.

But according to the claim, Willis Music gave permission to use the lyrics without consulting Mrs Newlin's heirs even though the book makes clear that Mrs Newlin was the author of and owned the copyright to the lyrics.