McDowall released his debut album Believe with Syco at the end of 2011, but it failed to hit the top of the charts and he was quickly dropped by Cowell's label.
Speaking to the Daily Record, McDowall suggested that it was his nationality that had hindered his relationship with Cowell and his music career. He also pointed to the elimination of Paige Turley from Monday night's (May 7) BGT semi-final as another example of Cowell's "anti-Scottish" behaviour.
"It's anti-Scottish Simon Cowell. You can print that if you want because I don't like him anymore," said the singer.
"He made positive comments that suggested he would give Paige the chance to come back - but he didn't stick by her. I wouldn't say I am Simon Cowell's friend."
He added: "I'm not performing on this year's show, which is bad considering I'm last year's winner. You'd think I'd get the chance to come back and perform like the winners usually do.
"He has said in the press that maybe I shouldn't have won last year's show, but tough. I did. It's one of these things that hasn't gone in his favour. The anti-Scottish thing is going to put Scots off him and could affect the popularity of Britain's Got Talent.
"Simon goes on about how the public choose the winner and they are always right. But he has gone against that by preventing me from appearing on the final."
Britain's Got Talent continues tonight at 7.30pm on ITV1.
Watch McDowall's 2011 audition for Britain's Got Talent below: