The 51-year-old quiz master felt personally insulted after the hopeful branded the latest addition to his team, Jenny Ryan, as "not very good".
Lucy, a 28-year-old Charity Worker from Leeds, was the first contestant playing this evening. When host Bradley Walsh asked her who she would like to face, she revealed that it was Jenny, affectionately known as The Vixen, saying: "I reckon her knowledge isn't that good.
However, Jenny was not available and Lucy found herself taking on Mark instead. Before playing against Lucy, Mark said: "Do you want to know how good The Vixen is? She's way better than you and I'm gonna whoop you for that comment. I got a message from backstage. Destroy her! Written in red lipstick.
Mark looked pleased, adding: "On behalf of both Jenny and myself, ta-ra. You insult one of us, you insult us all," prompting Bradley to say: "Don't mess with the chasers!"
The Chase: Mark Labbett loses his temper and SLAMS contestant for insulting Jenny Ryan
You insult one of us, you insult us all.
Viewers instantly took to social media, shocked that Lucy had "disrespected" the newest chaser.
One viewer tweeted: "How dare you disrespect the vixen, stunning and intelligent = perfect combo. Get her Beasty!#TheChase."
"@ITVChase Hate it when people are rude and disrespectful towards the Chasers; I am in awe of the Chasers' quizzing achievements! #TheChase," posted another.
A third said: "This girl reckons jenny "the vixen Ryan" isn't as good. Is she kidding? She's ace! #TheChase."
"#thechase #gangofchase starting to feel sorry for Jenny everyone thinks they can beat her and only like 97.6% can," noted a fourth.
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However, hordes of fans came out in support of Mark for standing up for his fellow quizzing colleague.
One viewer tweeted: "Good old @MarkLabbett sticking up for the lovely @jenlion #thechase #rudecontestant."
"Ooo she has p***ed Mark off. The beast defending my queen on #TheChase," posted another, as a third said: "Love a bit of#TeamChaser camaraderie #TheChase."
"@MarkLabbett I would NEVER bad-mouth a Chaser like that…. I'm in awe of all of you! #thechase," posted a fourth.