The talent show hopeful is said to have "humiliated" a junior production assistant after he was told he had exceeded his ticket allocation for guests.
"He must think he already is Jesus to be getting away with this sort of stuff," a source told The Sun.
"He is acting how a small man thinks a big star acts, but really he is making a proper show of himself - no big celebrities act like that. It's unprofessional.
"You don't see Lord Lloyd Webber screaming and throwing his toys out of the pram."
The insider continued: "It wasn't the first time he pulled that stunt but this time seats couldn't be found so he humiliated a member of staff and reduced her to tears.
"It was unsightly to watch and everyone is horrified."
Nathan James will compete for a place in Tuesday's (July 24) live semi-final during tonight's (July 23) episode, which airs at 9pm on ITV1.
Andrew Lloyd Webber recently placed an alcohol ban on the remaining contestants.
Watch Superstar contestants discuss their most embarrassing moments below: