The eight-year-old, who has been hailed as the real life Hermione Granger from Harry Potter, wowed the judging panel after performing a series of magic tricks that left them all speechless.
However, not everyone was impressed with Issy including fellow magician Carl Leek – known by his stage name The Starman – who claimed she had taken his stunt.
“It’s ludicrous. The routine is literally a carbon copy,” he said in a recent interview.
“My act was built around the T-shirt and I have emails from the producers sending me images of Simon I could legally use as my first printed T-shirt was refused,” he told The Sun.
Carl went on to say he felt “sick” after he watched the stunt, saying that it was like “looking in the mirror”.
The magician went on to claim that he had even been called to perform the act in front of Simon Cowell, David Walliams, Amanda Holden and Alesha Dixon back in February – before Issy’s audition.
He explained: “During my audition I received four yeses, got a standing ovation from the audience.”
Carl added: “My audition was in Birmingham a week before the Manchester auditions, so how can the judges be surprised by this trick?”
Nevertheless, he stated that Issy was “innocent in all of this” and that he was angry at Britain’s Got Talent and the show’s producers.
A Britain’s Got Talent spokesperson told Express.co.uk: “Issy Simpson came to ‘Britain’s Got Talent’ with her own routine and magic performance prepared, which she then duly performed at her audition.
“Any suggestion otherwise would be completely false and defamatory.”
Britain’s Got Talent continues tomorrow on ITV at 8pm.