Simon Cowell may soon have a new venue to go head-to-head with his longtime rival, American Idol creator Simon Fuller: Both are planning musicals for London’s West End. Cowell wants to build a stage show around his global franchise X Factor. Cowell has approached Brit comedian Harry Hill to write the book and comedy music producer Steve Brown — who has worked with UK X Factor finalist Rebecca Ferguson — to come up with musical ideas. Cowell’s production company Syco tells me that it’s still in “very early discussions” about the show. Hill fronts popular ITV comedy TV Burp. Its latest season starts airing soon — right before the current UK run of The X Factor. The comedian pokes fun at clips of the previous week’s TV shows, often singling out Cowell’s music contest. Brown wrote the music for TV Burp and has also written for Cowell’s regular UK TV presenters Ant’n’Dec, so it’s all very much in the family.

Fuller — who is suing for an executive producer credit on the U.S. version of X Factor — is a step ahead with his planned West End stage musical Viva Forever, based on the songs of his pop group the Spice Girls. Producer Judy Craymer, who dreamed up the worldwide Mamma Mia! phenomenon, begins workshops next month and hopes to open Viva Forever by March 2013. The show promises “a glimpse of the underbelly of TV talent shows.” Brit TV comedian Jennifer Saunders (Absolutely Fabulous) has written the book.

The problem both The X Factor and Viva Forever could face is finding space in a glutted West End. Only a few theaters can handle big musicals, unlike Broadway, where most venues are of a similar size. “West End theaters are rarely dark,” theater consultant Richard Andrews tells me. “As a result, theater owners can afford to be more choosey in the shows they take in, and also be more reluctant to book a show a long way in advance.”

There’s a lot of money at stake. X Factor is being produced in 29 countries, which could enable a stage version to open around the world. Mamma Mia! has just opened in China, having grossed more than $2 billion to date with 50 million people having paid to see it worldwide.

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