According to The Sun, one act auditioned in Manchester earlier this week with 'And I Am Telling You I'm Not Going', which Jennifer famously sang in the hit film 'Dreamgirls'.
But moments in to the audition, Simon called a halt - saying the track is "too heavily linked to another show".
The hopeful was apparently permitted to perform a further THREE tracks, although it is not clear whether they made it through to Bootcamp.
"X Factor and The Voice may both be on ITV now, but Simon still wants to be the best show and there’s real rivalry there," a source said.
"This one girl tried to sing And I Am Telling You, but Simon stopped her immediately and told her to sing something else.
"He did let her try another three tracks, so it’s not like he shot her down there and then."
Jennifer joined The Voice UK for its first series on ITV earlier this year, and eventually won with her act Mo Adeniran.
However, her future on the show is now up in the air - as she recently joined the American version.
The X Factor 2017 auditions haven't exactly got off to a smooth start, with two judges being forced to miss filming.
Nicole Scherzinger was briefly absent at the weekend due to a prior commitment, with Britain's Got Talent star Alesha Dixon temporarily replacing her.
The former Mis-Teeq member returned as a guest judge today, this time filling in for Sharon Osbourne, who has taken time off due to her back injury.