The retailer said the UK closures come as part of an overhaul which will affect around 100 stores as it looks to cut back on clothing and home while boosting its Simply Food chain.
Details of the restructure came as it said underlying pre-tax profits fell 18.6% to Â£231.3 million in the six months to October 1, while bottom-line profits crashed 88.4% to Â£25.1 million.
Chief executive Steve Rowe also outlined plans to shut 53 stores across 10 international markets - including 10 in China and seven in France, while pulling out of Belgium, Estonia, Hungary and Lithuania - putting around 2,100 jobs at risk.
M&S said it will close around 30 of its 304 full-line stores - selling clothing and home as well as food - and downsize or replace around 45 shops to Simply Food outlets.
It will also relocate some other outlets, while opening other stores in so-called under-served areas.
The group remained tight-lipped on the number of UK staff impacted by the plans, but said that, where possible, it would keep "job continuity" for affected employees.
The group insisted it would have more stores overall after the revamp, with already-announced aims to open more than 200 Simply Food outlets by the end of the 2018/19 financial year.
Mr Rowe, who took over from Dutchman Marc Bolland in April, said: "Over the next five years we will transform our UK estate with around 60 fewer clothing and home stores, whilst continuing to increase the number of our Simply Food stores.
"In the future, we will have more inspiring stores in places where customers want to shop."
He added: "These are tough decisions, but vital to building a future M&S that is simpler, more relevant, multi-channel and focused on delivering sustainable returns."
The latest restructure moves come as M&S revealed more sales falls in its embattled clothing division, with like-for-like sales down by 5.9% in the first half.
But it narrowed the sales decline from 8.9% in the first quarter - its worst performance for a decade - to 2.9% in the second quarter.
Same-store food sales fell 0.9% over the half-year, while it saw growth of 0.3% at M&S.com, leaving overall UK like-for-like sales 3% lower.