I Can't Sing!, which is written by Harry Hill and Steve Brown, opened in a storm of flashbulbs as X Factor faces including Louis Walsh, Stacey Solomon and boyband Union J mingled with celebrities including David Walliams and Amanda Holden on the red carpet.
Speaking to WhatsOnStage before the show, Cowell said: "The first preview I went to I was really nervous, but it was great, and I went back for another performance about ten days ago. The public reaction has been really encouraging. They're creating word of mouth, we haven't hyped this show. And whether it works or not, I love it."
In typical Harry Hill fashion, the show, which is directed by Sean Foley, is a madcap spin on The X Factor, replete with a singing hunchback, a Wagnerian chorus and talking dog named Barlow.
Photos by Dan Wooller for WhatsOnStage