Keira Knightley burst into laughter while filming sex scenes with her THE DUCHESS co-star DOMINIC COOPER - because he was forced to wear a skimpy pair of underwear.
Knightley insists she struggled to keep a straight face after the actor changed into the flesh-coloured garment, which made him appear naked on camera.
She tells MTV.co.uk, "I couldn't stop laughing. He's a very good-looking guy but he's standing there naked apart from a skin-coloured nappy (diaper).
"It's the most unsexy thing ever. The director even had to ask if I could be a little more supportive and I had to say 'no - he's wearing a nappy!'"