Brian, 36, was appearing on the Channel 4 show to promote his upcoming tour with former Boyzone star Keith Duffy, 42.
The pair have formed their own man band, Boyzlife, which sees them perform Westlife and Boyzone hits to a live audience.
Speaking to hosts Tim Lovejoy, 48, and Simon Rimmer, 53, Brian joked that they had to have a monitor with the lyrics to the songs they were performing because they had forgotten the words.
Keith explained: “I never sang the lead vocals back in the day so I never knew them. I’m learning them for the first time.”
Brian McFadden made an awkward gaffe while appearing on Sunday Brunch with Keith Duffy today
The pop stars spoke to Tim Lovejoy and Simon Rimmer about their upcoming tour as Boyzlife
We’re both anorexic
Attempting to make a joke, Brian added: “We can’t read we’re both anorexic.”
Evidently meaning to say “dyslexic”, the ex-husband of Vogue Williams carried on referencing the gaffe throughout the interview.
Those watching at home, however, were not impressed by the blunder.
One social media user fumed: “Give Brian a slap …he is not remotely amusing with his anorexic 'joke' #sundaybrunch.”
Brian repeatedly made a joke that he and Keith were "anorexic"
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“#SundayBrunch ohhhhh he means dyslexic not anorexic. What an absolute tit.” another added.
Someone else commented: “#SundayBrunch not sure what this joke is about being anorexic? Weird.”
“That awkward moment when Brian McFadden CONSTANTLY says “anorexic” when he means dyslexic. #SundayBrunch,” a fourth remarked.
Sunday Brunch continues next Sunday at 9.30am on Channel 4.