However, it was in fact 27-year-old Robbie Williams tribute act Dan Budd performing the crooner's hit, No Regrets.
Dan's career as a tribute artist first became successful when Robbie, 42, tweeted a picture of himself next to a poster of one of the lookalike’s upcoming gigs, almost three years ago.
“Good grief, you look like a friend of mine,” said Gary, before adding: “I’m scared to ask what you do for a living.”
One viewer tweeted the 46-year-old singer, asking: "@GaryBarlow why is @robbiewilliams performing on #LetItShine?"
Let It Shine fans gobsmacked as ROBBIE WILLIAMS auditions for Gary Barlow series
Good grief, you look like a friend of mine.
"Just turned on let it shine… for a minute I actually thought it was @robbiewilliams," posted another.
A third said: "Mum walks in thinking I'm legit watching Robbie Williams #LetItShine."
"@garybarlow OMG Dan Budd on #LetItShine! had to look up as thought Robbie was auditioning, then I did look up and still thought the same!!!," tweeted a fourth.
Other viewers praised the tribute act, but some noted that, in an awkward twist, the contestant was singing about Take That's break up.
Let It Shine: Robbie Williams poses in front of a poster for tribute act Dan Budd
The best musical talent shows
Wed, November 16, 2016
As Gary Barlow launches new talent show 'Let it Shine' to find the stars of a Take That musical, we look at the successful musical theatre talent shows of the past.
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Let it Shine – The new BBC show will see Gary Barlow looking for five young men to star in a new Take That musical
One viewer tweeted: "Can't believe the Robbie tribute on #LetItShine singing the song he wrote about take that's break up @GaryBarlow."
"Watching #LetItShine I can spot a fake @robbiewilliams a mile off but that man had it nailed," posted another.
A third joked: "That awkward moment when the Robbie Williams tribute act sounds better than actual Robbie Williams. #LetItShine."
"This dude on #LetItShine doesn't half sound like @robbiewilliams wow! Closed my eyes and thought I was listening to Robbie!! It's eerie. Every mannerism and the way he rolls his eyes is the same," posted a fourth.