Justin Bieber is threatening to sue bosses of a website who posted photos purporting to show unairbrushed images of the pop star from his recent Calvin Klein shoot.

The Baby hitmaker joined the advertising campaign and posed in his underwear for a series of photos that were unveiled this week (beg05Jan15), but further images were subsequently leaked online showing a much less hunky Bieber.

The snaps show the pop star with a much smaller chest and back, thinner arms and legs, a barely-there butt and a dramatically reduced bulge in his underpants, but Bieber is adamant the original photos have not been tampered with.

In a letter obtained by Tmz.com, his team threatens bosses of breatheheavy.com with legal action for posting the pictures, which they insist are fake.

They are asking the website owners to remove the photos.