The 37-year-old singer is helping judge Gary Barlow whittle the eight semi-finalists down to four, and left everyone speechless when he greeted them wearing nothing but his birthday suit and a few leaves to hide his modesty.
A source told The Sun, 'It was hilarious. Gary, the contestants and crew were stunned.
'Robbie just stood there stark naked apart from these tiny leaves.'
The prank is believed to have been Robbie's wife Ayda Field's idea.
'Ayda has a cheeky side and convinced Rob it would be a good laugh,' added the insider. 'He was immediately game.'
Robbie fans will be able to get an eyeful when the footage airs in the UK in two weeks' time. An X Factor spokeswoman said, 'Robbie clearly had nothing to hide from the hopefuls.'