Klass joins famous faces Twiggy, Erin O'Connor, Laura Bailey, Noemie Lenoir and Lizzy Jagger.
Steve Sharp, marketing director at M&S, said: "We're thrilled that Myleene has joined us for this campaign. She's a beautiful and very talented young woman with a lovely, warm personality."
The latest instalment of the award-winning campaign is again being handled by Rainey Kelly Campbell Roalfe/Y&R. The photographer is William Garrett and the TV ad is being directed by Joe Roman. The campaign breaks in March 2007.
Klass said: "Working with the other girls was like your first day at school. At first I was a little bit nervous... who wouldn't be standing next to Twiggy and Erin. But once I put on those clothes, and the way the whole team treat me, then I started to feel like a real M&S girl. Every woman in the UK sees the M&S ads and wants to be a part of it... it's the ultimate girl gang."