
Klass, who shot to stardom in 2001 as part of the television-created pop group Hear'Say, has been promoting the M&S brand since February 2007.
She has appeared in a number of high-profile campaigns for the retailer, including the recent Christmas TV spot, which also featured Take That, Lily Cole and Twiggy.
News of the new deal for Klass comes just four months after M&S made 1,230 staff redundant.