During the second day of filming at the London Palladium, the show's boss was left stunned by a singer and was overheard telling his fellow judges: "A star is born."
The act, who had a heartbreaking backstory, is one of the few performers not to get buzzed off by Cowell during the opening days of auditions.
However, the reality TV veteran was clearly moved by the performer, claiming that it was the "easiest" yes he'd ever given on a talent show.
After the act finished, Cowell added: "I don't want you to go."
A witness told Digital Spy: "It was a proper Susan Boyle-type moment. Simon had been buzzing off people all day and then one act turned things on their head.
"The whole audience were moved and on their feet at the end."
Viewers will have to wait for the spring to find out who the act is, and if they impressed the other judges and made it through to the second stage of the competition.
Watch our interview with Simon Cowell at the launch of Britain's Got Talent below: