Marks & Spencer is closing one in three of its core clothing and home branches in a dramatic retreat from the UK high street that will trigger thousands of job losses.

M&S on Tuesday revealed plans to close a total of 100 shops by 2022, as it expanded an existing store closure programme. It said the closures were vital for the future of the struggling retail business but Usdaw, the shopworkers’ union, accused M&S of “salami slicing”, as staff came to terms with a third wave of store closures.

The retailer has embarked on a “radical transformation” plan intended to tackle years of falling sales and profits. M&S has confirmed the locations of only 36 store closures so far, including the identities of a further 14 sites that were revealed on Tuesday as part of the enlarged programme. The cull began 18 months ago when the chief executive, Steve Rowe, said he would slash the amount of shopfloor space devoted to M&S’s clothing ranges.

Under the expanded programmes, M&S is eliminating 25% of the floor space devoted to clothing and homewares. M&S has 1,035 stores in the UK, 300 of which sell clothing, homeware and food. There are around 700 Simply Food convenience stores.

Sacha Berendji, the Marks & Spencer retail, operations and property director, said the company was trying to make its store estate “more relevant” and better able to support the growth of its website.

With another 64 branches still to be earmarked for closure under the cuts, the process is creating huge uncertainty for staff. So far the programme has affected 1,500 out of 72,000 UK shop staff.

“Closing stores isn’t easy but it is vital for the future of M&S,” Berendji said. “Where we have closed stores, we are seeing an encouraging number of customers moving to nearby stores … which is why we’re continuing to transform our estate with pace.”

Three stores – Bayswater, Fleetwood outlet and Newton Abbot outlet – will close by the end of July while Holloway Road in London and Clacton-on-Sea in Essex will close by early next year as they are replaced by nearby food-only outlets.

Darlington, East Kilbride, Falkirk, Kettering, Newmarket, New Mersey Speke, Northampton, Stockton and Walsall are proposed for closure and will enter a period of consultation with employees. M&S said it would aim to redeploy staff before considering redundancies.

M&S’s announcement comes as UK retailers line up a wave of closures amid tough trading conditions and rising costs. Last week Mothercare announced plans to shut 50 stores, putting at least 800 jobs at risk. Fashion chain New Look is shutting 60 outlets and House of Fraser is also planning at least 20 closures.

Outside the Holloway Road store on Tuesday, which first opened its doors in 1914, shoppers reacted with shock. The once grand frontage, complete with classical pillars, is pockmarked by holes from previous signage, the windows grimy with old paint and the interior dated. But there was a steady flow of shoppers browsing the clothes as well as the food aisles.

Margaret Late, 68, said she had been visiting the store for about 40 years. “I come down every week but I rarely buy anything as I find it too expensive. I do like to get my underwear here and sometimes my husband’s jeans but I prefer Bonmarche for clothes. It’s cheaper and nicer stuff. It’s a shame for M&S to go, though. It will probably end up as a pound shop.”

Sofya Gaal, 39, said she had been using the store for most of the 22 years she has lived nearby. “It’s a shame it’s closing, it’s probably because of online. I shop online but I like to come out to the shop and get some fresh air. I probably come here three or four times a week.”

The Usdaw national officer David Gill said: “Staff in all M&S stores were extremely concerned about what the future holds when the company announced a wide-ranging store reorganisation in November 2016.

“At the time they did not say where the axe will fall by detailing which stores are set to close,” said Gill. “Today we’ve seen a third wave of closures announced, which is devastating news for the staff in those stores and the uncertainty continues for everyone else. This salami slicing approach to reorganising the business is extremely distressing for the staff.”

On Wednesday M&S is expected to report pretax profits of £573m, down from £614m in 2017, marking the second consecutive year of declining profits and falling well short of the peak of £1bn reported in 2008. Sales of clothing and home products at established stores are forecast to be down 1.1%, and food sales down 0.2%.

One retail consultancy said M&S was in danger of losing its crown as the UK’s largest clothing retailer.

‘‘Marks & Spencer has dominated the UK clothing market for decades, but its lead as number one is perilously close to being lost to Primark this year,” said Maureen Hinton, a research director at GlobalData. “The closure of yet more stores will hasten the decline unless it can shift the lost sales to its online channel and transfer to its other stores.”

In 1997 M&S achieved its peak clothing market share of 13.5% in the UK but the past two decades have seen the lead chipped away to 7.6% just ahead of Primark on 7%, according to figures from GlobalData.