The 34-year-old TV presenter will initially launch a fashion collection for Littlewoods, before promoting swimsuit and lingerie ranges later this year.
Klass has worked for M&S for five years in a promotional capacity.
Her campaigns for the retailer included its summer ad campaign with Danni Minogue and Gary Barlow.
It is unclear whether Klass was axed by M&S or whether she left of her own volition.
She was unveiled as the new Littlewoods brand ambassador when she posed with a cheetah at a photoshoot.
Littlewoods owner Shop Direct Group believes Klass represents a good fit for the brand, particularly as it believes she is perceived as a hard-working mum.
Littlewoods has in the past worked with Coleen Rooney to promote its goods. The retailer said it would continue to work with Rooney and Klass.
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