Jason Donovan and Mel C are to help musical maestro Andrew Lloyd Webber find his lead for the new production of Jesus Christ Superstar.
The Australian actor and former Spice Girl were selected just days after the theatre impresario suggested he may eventually cast a woman as the main part.
The pair will assist with "callbacks" - in which the best auditionees are whittled down to a final 40 - before working with the successful applicants during intensive training sessions.
It is not the first time the duo have worked alongside the multi-millionaire composer.
While Donovan took the lead role in Joseph And The Amazing Technicolor Dreamcoat, Mel C was brought in to offer advice to contestants hoping to play Dorothy in Lloyd Webber's production of The Wizard Of Oz.
The singer has also taken to the stage, performing in the long-running West End production of Blood Brothers.
A spokesman for ITV, which is screening the search for a star, said: "Andrew Lloyd Webber has invited Melanie C and Jason Donovan to assist him at callbacks, which starts today for three days.
"The panel of judges for the live shows are still to be announced."
Auditions have been taking place in London, Dublin, Belfast, Manchester, Glasgow and Cardiff, with the most promising new talent performing on stage in front of Lloyd Webber and judges.
The winner will have a lead role in a tour of the rock opera, which will open at London's O2 Arena.