Speaking to Absolute Radio DJ Christian O'Connell's Beer and Cake Club, Decker said: "I just felt like I'd been kicked in the stomach. I was speechless.
"I just thought it was unnecessary and completely untrue, I don't sing out of tune. In fact, I'm not perfect but I find it quite difficult to sing out of tune, I've got very good relative pitch if you want to be boring and trainspottery about it.
"So if he was looking to be controversial because he's in the Simon Cowell chair he just picked the wrong thing to say. But then I could not get over how many people leapt to my defence, and my Twitter following went from 9,000 to 31,000 in about an hour."
She continued: "I'm surprised at him because he is an excellent songwriter and a really, really good musician, so he knows that what he said isn't true, that's the first thing.
"I'm known for that song, I'm known for my big voice and my vocal performance. So he sort of like trashed everything I'm respected for in a second, and it was a very peculiar thing for him to say."
Asked if Barlow was playing up to the cameras, Decker said: "If he was he was silly. It was the wrong thing to say.
"If it popped into his head he was daft, if he was primed by the show he should have had more sense, you know, but there you go."
Decker, who performed Take That's 'Back For Good' at the Leicester Square Theatre, also suggested doing a duet with Barlow so that he could see "how well" she sings.