The retailer, which has already shut 21 outlets included in the figure, said it had identified a further 14 clothing and home stores to be shut in its current financial year.
The development added to the sense of crisis facing the high street as names such as Toys R Us are lost and others, including Mothercare most recently, move to secure their futures.
The high street slowdown has coincided with the latest turnaround effort at M&S.
The company said all 626 people affected by the latest 14 proposed closures would be offered roles elsewhere before redundancy was considered.
The retailer listed the 14 locations – saying its Bayswater store and Fleetwood and Newton Abbot outlets would shut their doors for the final time by the end of July.
Its Clacton and London Holloway Road branches were to close by early 2019, M&S said, to coincide with new food stores opening nearby.
The other sites affected are Darlington, East Kilbride, Falkirk, Kettering, Newmarket, New Mersey Speke, Northampton, Stockton and Walsall.
M&S, which is due to update the City on its trading performance on Wednesday, is undergoing a major shake-up on the direction of chief executive Steve Rowe and chairman Archie Norman.
The programme has seen it invest in its online offering at the expense of its costly clothing and home store estate as shopping habits shift towards digital channels and customers seek value.
The chain currently has 300 clothing and home stores in the UK and just shy of 600 Simply Food outlets – the majority franchise-owned.
It has also been re-organising its distribution network to better serve its revamped store and online priorities while there are new faces trying to turn around its struggling fashion business – particularly womenswear.
Retail and property director, Sacha Berendji, said: “We are making good progress with our plans to reshape our store estate to be more relevant to our customers and support our online growth plans.
“Closing stores isn’t easy but it is vital for the future of M&S.
“Where we have closed stores, we are seeing an encouraging number of customers moving to nearby stores and enjoying shopping with us in a better environment, which is why we’re continuing to transform our estate with pace.”
The M&S transformation plan is ultimately aimed at growing profitability and therefore value for investors after a period of decline that has seen its market capitalisation slip below that of online fashion rival Asos.
Shares – down 4% this year amid the troubled economy for retailers – were the biggest fallers on the FTSE 100 after the announcement, falling more than 2%.
Its annual results are tipped by analysts to show a fall in the company’s trading profits.
David Gill, national officer for Usdaw, took aim at what the shopworkers’ union saw as a “salami-slicing approach” under the transformation.
He added: “Usdaw has thousands of members working for Marks & Spencer and the staff now need, more than ever, the representation and support of an independent trade union.
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“We again urge M&S management to abandon their long-held resistance to recognising Usdaw as the union to represent its staff.
“It is simply unfair that they continue to refuse their staff access to Usdaw, having made the decision on their behalf not to engage with a trade union.”