Jack Black is desperate to lose weight so he can star in more nude scenes.

The 38-year-old actor - who has just become a father for the second time - is determined to shed his excess pounds to be more attractive to his fans when he sheds his clothes on the silver screen.

He said: "When I saw my rear end in my nude scene in Margot at the Wedding, it actually had a double chin. I’ve joined a gym because the next time you see me naked I’ll be trim."

Jack also said he wanted to slim down because he was getting worried about his health.

He added to America’s OK! magazine: "I want to feel good too. Now if I eat a cheeseburger and fries, it tastes so good at the time and then right after it’s like, 'What did I do? Why?’"

Jack - who unwittingly confirmed his Kung Fu Panda co-star Angelina Jolie was expecting twins recently - revealed he used to be a martial arts enthusiast.

He said: "When I was a kid, I took karate and judo classes. I even won a trophy in a judo tournament, but I confess I outweighed the competition by 20lbs.

"I never took any kung-fu classes though, I just saw it on TV and in the movies."