Bostock is leaving M&S after eight years "by mutual consent", to pursue other interests, effective 1 October.
She will be replaced by John Dixon, who has been with the company for 26 years, and is currently the executive director of food. Dixon will report directly to chief executive Marc Bolland.
Two new positions have been created as part of the changes by Bolland. Belinda Earl, previously the chief executive of Debenhams, Jaeger and Aquascutum, will take up the position of style director on 1 September, reporting to Dixon and working two to three days a week.
In addition, Patrick Bousquet-Chavanne, the former group president of The Estée Lauder Companies, will take on the new role of corporate director of strategy implementation and business development from 1 September.
Earl's role will involve her spending two to three days each week focusing on product, while Bousquet-Chavanne will be developing strategic initiatives and new concepts across the company.
Dixon's position will be filled by Steve Rowe, currently director of retail.
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