The star has been left “heartbroken”, after the 89-year-old passed away on Sunday (5 July), after suffering a stroke.
Simon Cowell with his mum Julie and brother Nick
Simon’s close friend Sinitta and new ‘X Factor’ host Olly Murs have led the tributes to Julie on Twitter, after it was announced that auditions for the show had also been postponed as a result of Simon’s bereavement.
Announcing Julie's death last night, the family said they were heartbroken and would "miss her terribly".
"Today our beloved mum Julie passed away. Anyone who had met her would know what a truly special and kind lady she was. We are heartbroken and will miss her terribly, but she is now at peace," a statement said.
"We would like to thank the media for kindly respecting Julie's privacy throughout and we politely request continued privacy for the entire family at this particularly difficult time."
Julie was the mother of four boys - Nicholas, Simon, Tony and Michael, and step-mother to John and June.
It is currently unknown if ‘X Factor’ auditions will go ahead in Manchester, on Tuesday, as planned.