He will remain in the chairman role until he leaves the retail business in March 2011.
Sir Stuart will continue to chair the M&S Board and will work alongside new chief executive Marc Bolland when he joins the company on 1 May.
M&S has appointed headhunters to recruit an independent chairman to replace him.
Sir David Michels, Deputy Chairman of Marks & Spencer, says: “The Board has set out this process to ensure a smooth transition over the coming months and enable Marc to draw on Stuart’s considerable experience.
“We are pleased to be moving into the final stage of our commitment to split the roles and appoint an independent Chairman by March 2011.”
Bolland was poached from supermarket chain Morrisons in November.