Fleur sang Mark Ronson’s Uptown Funk on Saturday, before Mark’s album had even been released.
This angered Lily, who wrote on Twitter: “Is this even out yet? #XFactor … Well done to @sony for managing to get a not-yet-released song of theirs sung on prime time weekend telly.
“It is an amazing song and Fleur did it justice but it’s still a great example of music industry corruption.”
But Simon hit back, saying that he had turned down Lily for a judging role on the ITV show.
Fleur’s rendition of Uptown Funk in Saturday night’s semi-final went down so well that within hours she was at the top of the iTunes chart.
The success of the tune – before the original had even gone on sale – prompted Ronson’s label to rush release the track, featuring Bruno Mars, five weeks early.