The accusation came after Cowell's team Emblem 3 sung One Direction's song "What makes you beautiful", reports dailymail.co.uk.
Lovato, 20, who shares the judge's panel with Cowell, had a war of words with him on the show aired Nov 7 and said to Emblem3: "If I had been your mentor, I would have made sure that I wouldn't have plugged one of my artists that you can easily be compared to."
Cowell, 52, defended himself by saying it was a tribute to One Direction.
"It was a hint of One Direction. It was a tribute. That's called being clever," Cowell said.