Bear Grylls accidentally bared all in a text to showbiz buddy Dermot O'Leary.
The broadcaster, who has starred in several survival shows and became chief scout in 2009, told Piers Morgan he sent the picture before skinny-dipping in the sea off Wales.
Appearing on his Life Stories show tonight, he said: " We own this little island in north Wales and there's a great cliff jump into the sea and I do this every morning to be honest when I'm up there and before anyone's out.
"But the thing is with this photograph, I looked at it on my iPhone and when I texted it looked fine, sent it to him. And actually if you do blow it up, you can just see the end of what I didn't realise was visible so - yes, that's the danger of small screens on iPhones.
"Can I say it was an incredibly cold winter's day a week ago."
Bear, who also proposed to his wife while swimming naked, admitted: " If there's a substantial body of water around and nobody's looking, I'm often quite tempted to strip off and dive in."
:: The full interview is on P iers Morgan's Life Stories: Bear Grylls on ITV at 9pm tonight.