The TV star - who is known as 'The Beast' on the quiz show - has been shedding the pounds for a while and has lost an incredible 10 stone so far.
Updating his fans on Twitter, Labbett shared that all his efforts were paying off as he has gone down some clothes sizes.
"Just got 18.5 inch work shirts from the lovely people at @DoubleTWOshirts (and some 19s)," he tweeted.
"Considering I used to be 22 inch collar, I am a very happy bunny :)."
Just got 18.5 inch work shirts from the lovely people at @DoubleTWOshirts (and some 19s). Considering I used to be 22 inch collar, I am a very happy bunny :)
— Mark Labbett (@MarkLabbett) March 16, 2023
Fans congratulated the star on how well he was doing in his bid to get in shape, saying he was looking "great".
"Well done," praised one fan, adding a clapping emoji.
"Awesome Mark, you are doing amazing!!" another person tweeted.
Somebody else said: "Well done Mark! Keep it up!"
"Weight loss will not make any difference to your magnificent recall of general knowledge," remarked another of the TV star's fans.
"You are a top man and respected.
"Good luck in getting to your personal target weight."
The quiz champion, 57, has previously revealed that he weighed 29 stone at his heaviest and that he was diagnosed with diabetes a few years ago.
And in 2021 he told The Sun that he had dropped 10 stone and that he was the lightest he had been in 25 years.
As he started to get trim, Labbett jokingly referred to himself as "skinny beast" on Instagram.
Watch: Gloria Hunniford criticised for Mark Labbett 'belly' comment