The star revealed he was eating just a fifth of his daily calories during his time on the ITV reality series.
Opera singer Russell noted he couldn’t even keep his trousers up because of his waist shrinking.
And now he needs to use TWO belts to ensure they stay around his middle!
He told The Sun that the jungle diet saw him lose a whopping 20lb in 13 days.
Russell, 54, entered the camp at Gwrych Castle with musical star Ruthie Henshall on day four of the series.
The weight literally fell off me.
He left just under two weeks later alongside EastEnders actress Jessica Plummer in a double elimination vote off.
Russell explained to the tabloid: “I was mainly wearing the leggings for the first few days.
“But once I put my red pants on they were falling down my backside. I lost nearly 20lbs in two weeks in there, the weight literally fell off me.
“I was quite pleased with that, I was a big unit when I went in there, so I put my trousers on today and they fell down, I had to put two belts on to keep them up.”
Russell Watson’s fitness journey during lockdown
He previously told the newspaper he had already shed some timber after improving his fitness during lockdown.
Russell said before going on I’m A Celebrity: “The only good thing to come out of lockdown and all of this is the opportunity to get fit. I’m constantly on the road touring. So instead of a Caesar salad with some ham on the side it’s a Double Decker and a packet of Hula Hoops.
“Now I’ve been training every day but I’ve not set a goal. Last weekend I did 23 miles on my pushbike, when I first got on I did about seven minutes and it felt like my quads would explode, I was walking like Elvis Presley.”
Russell also keeps fit by playing tennis, boxing and working out on a Peloton.
Wonder which campmates have shed the largest amounts over the years? Find out more about I’m A Celebrity biggest weight losses and the stars who’ve dropped the most pounds in the jungle.
I’m a Celebrity… Get Me Out of Here! airs on ITV tonight, Thursday December 3, at 9pm.
Leave us a comment on our Facebook page @EntertainmentDailyFix and let us know what you think of this story.