The pair were looking back at their early careers on Big Brother, showing a clip of Alison appearing as a contestant on the show during its third season in 2002 before airing a video of Dermot hosting its spin-off show, Big Brother's Little Brother, which he presented from 2001.
WATCH: Dermot O'Leary apologises after swearing live on-air
Alison remarked: "You were so good on that show," to which her co-host jokingly responded: "Just on that show?"
The mum-of-one replied: "Yeah, just on that one," causing the pair to erupt with laughter.
"I'm only joking," she added, before Dermot quipped: "Such a [expletive]."
A shocked-looking Alison quickly addressed the viewers, saying: "Oops! Our apologies for that language."
Dermot sheepishly added: "Sorry," before swiftly moving on to the next segment.
Dermot let slip a swear word on Monday's show
Viewers were quick to react to the hilarious moment, with many taking to Twitter to express their surprise. One person tweeted: "Omg did Dermot just call Alison a [expletive] on #ThisMorning," while another added: "Did Dermot really just say that [laughing face emoji]."
Others pointed out the great chemistry between Alison and Dermot, who seemingly forgot that he was live on-air for a moment.
One person wrote: "Dermot O'Leary swearing on #ThisMorning [laughing emojis] brilliant. Alison and Dermot's chemistry is 100%!" while another added: "Hahaha such a [expletive]. Something I'd say - it would just slip out my mouth too."
Viewers praised the pair's chemistry
A third fan commented: "I'm screaming at Alison trying to keep a straight face after Dermot called her a [expletive] live on air," while another agreed, adding: "Alison trying not to laugh at Dermot is killing me."
The humourous moment comes just a month after ITV issued a statement addressing rumours of a feud between the two hosts.
An ITV spokesperson told Metro.co.uk: "Despite the challenges of starting their on-screen partnership during a pandemic, Alison and Dermot have developed a firm friendship and their chemistry has proved a ratings success.
"Since the pandemic ended, the pair are now able to enjoy socialising together post the show and have enjoyed spending time with each other's families too."
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