Justin Timberlake's mum once caught him having sex.

The Friends With Benefits star - who recently split from long-term girlfriend Jessica Biel - explained he was "too young" to be sexually active, so his mother wasn't pleased to discover him in bed with a girl.

He said: "I was caught one time. My mum wasn't cool about it. I was too young to be in bed with a girl, so she was upset."

In new movie Friends With Benefits Justin and co-star Mila Kunis play pals who vow only to have a physical relationship, but the Black Swan actress doesn't think she could have "sex without an ounce of feeling" like her alter-ego.

Mila added to US Elle magazine: "Ultimately, it ends when someone wants to go and get serious with somebody. More times than not, a person catches feelings and somebody gets hurt.

"When a female orgasms, a hormone gets released. I've never met a girl who can have sex without an ounce of feeling."

Meanwhile, Justin explained he is trying to cut down on the amount of time his bottom appears in the movie, as he is concerned what his mother will think.

He joked: "I'll be honest and say, like, I'm still trying to get into the editing room and cut down on my ass time. I'm like, 'Oh my God, my mum's gonna see that!'"

- BANG! Showbiz