Justin Bieber's mother is more impressed by him being a Calvin Klein model than a pop star.

While Pattie Mallette, 39, is proud of all aspects of her son's career, she thinks his being picked to star with model Lara Stone in a series of underwear adverts for the fashion brand sets a new benchmark for his fame and success.

Speaking at the filming of Justin Bieber's Comedy Central Roast, she told BANG Showbiz: ''I just think Justin is a handsome fellow. Never mind him being one of the biggest pop stars on the planet, my son is now a Calvin Klein model! That, to me, is absolutely crazy.''

Pattie also admitted she was ''a mixture of nerves and excitement'' before the comedy event, which saw a host of stars including Snoop Dogg, Ludacris and comedian Chris D'Elia take it in turns to make harsh jokes about the 21-year-old singer.

Topics on the night included his ex-girlfriend Selena Gomez, his numerous cringe-inducing appearances on 'The Ellen Degeneres Show', his bad driving skills and his abandonment of his pet monkey, Mally, in Germany.

The Roast of Justin Bieber will air on Comedy Central in the US on March 30 and on Comedy Central UK at 10pm on March 31.