McDowall, 33, won the ITV talent show in 2011, beating bookies’ favourite Ronan Parke.
Cowell, 59, didn’t hold back in his assessment of the star the following year, telling ITV’s Daybreak that McDowall “wasn’t one of our best ones.”
“I was disappointed with Jai,” he admitted. “You want to find someone who is a star all over the world and who has got a long career. With respect to him, he wasn’t one of our best ones.
He added: “You want a Diversity or a George Sampson or a Susan Boyle, a different kind of artist who is a genuine star.”
McDowall’s debut album was released on Cowell’s Syco label but only charted at number 54. He later parted ways with the company.
Ahead of McDowall’s appearance on Britain’s Got Talent spin-off The Champions, which sees former contestants go head to head, Cowell has admitted that he should have apologised to the singer “some time ago.”
“I did feel when I saw him again that he’s the definition of a proud man,” Cowell told The Sun. “He didn’t sulk - he just came back to prove a point. And he did.”
“I knew I had to apologise to him at that moment. It gave me a chance to say something publicly I should have said some time ago.
“But it as also the fact that he had the guts to come back and compete knowing I wasn’t that supportive the first time he was on the show.”
Britain's Got Talent: The Champions trailer
In the second episode of Britain’s Got Talent: The Champions, which will air tomorrow, McDowall receives a standing ovation from Cowell, The Sun reports.
Dance troupe MerseyGirls will also return to the stage, alongside 2018 winner Lost Voice Guy.
Britain’s Got Talent: The Champions continues on ITV at 8.30pm on September 7.