The Take That star urged Tulisa Contostavlos's girl group to give more prominence to their "best singer" Perrie Edwards after feeling underwhelmed by their performance of 'You Keep Me Hangin' On' by The Supremes on last night's show.
"Perrie is actually the best singer in this band," he stated. "I think she should be featured a lot more than she is. That's what missing at the moment for me, a focus, a lead singer."
Contostavlos responded by telling her co-judge: "What Little Mix represent are four different people who when they come together are four strong individuals. They love each other and want to give each other as much time on the record."
Barlow continued to defend his comments on The Xtra Factor, saying: "I have an opinion, whether you agree with that or not is regardless. Perrie is the best singer in that band and tonight they needed her.
"This week was actually Little Mix's weakest week for vocals. They needed her tonight more than they have ever done."
Band member Jade Thirlwall later told Caroline Flack and Olly Murs that even though she thought Barlow had been overly critical, the band would be taking stock of his suggestions.
"I think he was a little bit on the harsh side tonight," she said. "But you take everything that [the judges] say on board because at the end of the day they're the professionals. You've got to listen to them, go back and work on it next week."
Little Mix's second song 'If I Were A Boy' by Beyoncé was also panned by judges, prompting Contostavlos to bang her hands on the table and unleash an impassioned plea for viewers to vote.
"I got angry," she admitted on The Xtra Factor. "I'm not gonna lie. I think that last performance was amazing. How dare they put down my little muffins?"