Ellison will take over from Katherine Kingsley as the silent movie siren with a voice never meant for the 'talkies', Lina Lamont.
Cooper will continue to lead the company as Don Lockwood, joined by new cast members Louise Bowden playing Kathy Selden and Stephane Annelli who will play Cosmo Brown, completing the trio made famous by Gene Kelly, Debbie Reynolds and Donald O'Conner in 1952 MGM musical film.
The new cast also features Mark Hadfield, David Lucas and Frankie Jenna with Karen Aspinall, Emma Caffrey, Matthew Croke, Brendan Cull, Adam Denman, Kelly Ewins-Prouse, Charlene Ford, Olivia Fines, Tim Hodges, Nia Jermin, Peter Le Brun, Matthew Malthouse, Adam Margilewski, Oliver Metzler, Gillian Parkhouse, Joseph Prouse, Charlotte Scott, Zara Warren and Nancy Wei George.
The producers of the show said in a joint statement: "We are thrilled to welcome our new cast members to the Palace Theatre and looking forward to their first performance next month. It has been a joy to have brought this Chichester Festival Theatre production to the West End where we are delighted to be entering our second year."
Ellison, whose previous stage credits include Legally Blonde the Musical, Calendar Girls and Boeing Boeing, said: "Singin in the Rain' is one of my absolute favourite movie musicals, I watched it a lot growing up. When I saw the show I left with the biggest grin on my face and a huge spring in my step. Playing any West End role is a thrill and a privilege but to be playing Lina Lamont at the Palace Theatre in this show is pretty much as good as it gets."
Louise Bowden, who takes over from Whatsonstage.com Award nominee Scarlett Strallen, has previously appeared in White Christmas, Mary Poppins and Mamma Mia! on tour in the UK, A Midsummer Night's Dream and Ragtime for the Open Air Theatre, We Will Rock You at the Dominion Theatre and Guys and Dolls at the Piccadilly Theatre.
Stephane Annelli, who steps into Daniel Crossley's shoes as Cosmo Brown, has previously been seen in the West End in Legally Blonde the Musical, Saturday Night Fever and Sinatra. He recently appeared in Ragtime and A Midsummer Night's Dream at the Open Air Theatre and in On the Town at English National Opera.
Singin' in the Rain features classic songs by Nacio Herb Brown and Arthur Freed, including "Make 'em Laugh", "Good Morning", "Moses Supposes" and "Singin' in the Rain". Original screenplay and adaptations are by Betty Comden and Adolph Green. With choreography by Andrew Wright, the production is designed by Simon Higlett, with sound by Matt McKenzie and lighting by Tim Mitchell.
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