The former Spice Girls singer and the musical theatre star will assist Lloyd Webber at callbacks for the ITV show, which begin today (March 12) and are due to continue for three days.
The trio will whittle down the remaining hopefuls to a final 40 and continue to judge them at a subsequent "intensive training" session.
The panel for the Superstar live shows will be announced at a later date.
Speaking about the contest last week, Lloyd Webber declared that the audition stages have been "extraordinary".
"We had a boy from New Zealand, a boy from Israel, we had somebody coming in from Lithuania," he teased. "We've had 3,000 people in to audition for the role. We've even had a few girls... [and] one girl who's very, very good."
The 63-year-old also said that the eventual winner of Superstar will have to possess "enormous charisma".
"He has to be a genuine, genuine rock tenor," he explained. "Really, think Freddie Mercury, I mean that's the kind of range we're talking about.
"Whoever plays this role will be opening at the O2, and there are going to be big stars alongside. It's not just going to be any old casting."
> Jesus Christ Superstar the reality show: Bonkers or brilliant?