M&S has announced it will close 30 UK clothing and homeware shops and convert dozens more into food stores.
Chief executive Steve Rowe's turnaround plans will also see it open 200 new Simply Food stores as it shifts away from disappointing fashion sales.
He said M&S also planned to close loss-making shops in 10 international markets, including China and France.
The announcements came as M&S reported falling sales and profits in the six months to the end of September.
Mr Rowe said of the UK store closures: "This is about building a sustainable, more profitable business that's relevant for our customers in a digital shopping age."
He would not be drawn on job losses or which stores would close, amid warnings from trade unions that staff would be "extremely concerned" about where the axe will fall.
In total M&S said it would have about 60 fewer clothing and homeware stores in five years' time.
However, with new food outlets opening, the company will have more stores overall and "more towns will have an M&S", Mr Rowe told the BBC.
He added that customers still "love" M&S, but that it could do better.
The retailer has over 300 full-range sites, which sell clothing, homeware and food, and nearly 600 Simply Food shops in the UK.
It also plans to shut 53 international stores, including all 10 in China, half of its stores in France and all its shops in Belgium, Estonia, Hungary, Lithuania, the Netherlands, Poland, Romania and Slovakia.
That represents more than a quarter of the international stores that M&S fully owns, and will cost £150m-£200m. The company said it would start a consultation with 2,100 employees over the proposals.