She will begin performances April 26, with Van Outen departing the company April 23, at London's Savoy Theatre.
Casey, who is best known on television for playing Donna in the BBC's "Two Pints of Lager and a Packet of Crisps," has been seen onstage in The Invisible Man at the Menier Chocolate Factory, Oklahoma! at Chichester Festival Theatre and Well at the Apollo Theatre and Fame at the Shaftesbury Theatre.
She joins Susan McFadden as Elle Woods, Alex Gaumond as Emmett, Carley Stenson as Margot, Nicola Brazil as Serena, Siobhan Dillon as Vivienne and Peter Davison as Professor Callahan.
Legally Blonde the Musical is currently booking to March 31, 2012. To book tickets, contact the box office on 0844 871 7687, or visit www.legallyblondethemusical.co.uk.
