Barry Weissler, who co-produces the revival with wife Fran, told Star that Simpson began rehearsals in an NYC studio on Monday. "Yes, it is true. Ashlee Simpson is joining the cast of Chicago in London. We are very excited and are looking forward to gauging the London audience reaction." !

The Ashlee-Chicago deal, which was rumored to be in the works in recent weeks, was finalized this past weekend, the Simpson source confirms to Star. Ashlee is slated to begin starring in the musical in October in London's West End.

"Ashlee's a dancer; her background is in dance," the source tells Star. "She wants to do live theater. This will put all of those critics to rest. She's always wanted to be on stage, but she got diverted when she started selling so many albums."

Meanwhile, the Simpson source tells Star that an official announcement will be made in coming weeks, likely out of London, and that Ashlee will be photographed for the musical's posters in the coming days.