Marks & Spencer is increasing its stocks of smaller sizes in womenswear after a surge in demand left gaps on shelves.

Stuart Machin, the chief executive of the high street stalwart, said that three years ago about a fifth (21%) of the items sold from its main seasonal collection were size six to 10 and that had now risen to 35% as its “fashion and style ranges [were] resonating with a different customer group”.

Even outside those more fashionable ranges, sales of smaller sizes now represent about 23% of the mix, up from about 20% three years ago.

Answering a complaint from a shopper at the retailer’s annual general meeting – that there were not enough smaller sizes in stock – Machin said on Tuesday that he had personally looked at the orders on smaller sizes to ensure there were more and “if we sell out in autumn winter we’ll be doing very well”.

The demand for smaller sizes reflects the increasing popularity of M&S, once mainly worn by those well over 50, with shoppers in their 20s, 30s and 40s who, on average tend to be slimmer.

A spokesperson for M&S, which stocks women’s clothing from size six to 24, said: “We’re not moving away from larger sizes, it’s more about getting better at responding to demand and making sure we have the right size mix and availability for all of our customers – which is something we haven’t always got right.”

In May, Machin said more under-30s were buying M&S’s lingerie than ever before, helping take its market share in that product category to a new high of 38%.

That broader appeal has help the revitalised retailer win over 1 million more shoppers and boost profits by 41% in the year to 30 March.

The retailer has tried to improve its fashion credentials and reach a new generation of shoppers by using social media, including training its own staff as influencers. It has also enlisted younger names than it has traditionally used in advertising campaigns, such as Sophie Ellis-Bextor and Zawe Ashton.

Tie-ups with celebrities such as Sienna Miller, the 42-year-old actor known for her style, have also been important. M&S said 10 was the most popular size in Miller’s range and 32% of items in its spring and summer campaign were in size six to 10.

On the M&S website, six of the 19 clothing items available in Miller’s collection are sold out in sizes six to 10

Pippa Stephens, senior apparel analyst at GlobalData, said M&S’s decision to start stocking outside brands such as Nobody’s Child and Sweaty Betty on its website had helped boost its appeal with younger shoppers while the design of its own-label gear now had “more fashionable shapes and prints”.

But she said that it may also be that a larger number of older shoppers were now more trim and “more aware of exercise and healthy eating”.

Machin has also rejigged M&S’s food stores so that they have greater appeal to young families, with the Essential range of cheaper basics, larger packs and enough items to make it possible to do a full weekly shop.

Dozens of investors attended the meeting at the Paddington headquarters in person but this was many fewer than the vast gatherings the FTSE-listed company was once known for when shoppers often berated it over the length of sleeves, knicker elastic and other matters.

During the pandemic, M&S switched to holding the meetings online, but said it was now committed to a hybrid set-up.

At the meeting, the retailer had to defend its use of agency workers, such as cleaners, who are not guaranteed at least the real living wage unlike the main shop floor staff who are directly employed by M&S.

Machin and chairman Archie Norman also defended a decision to cut back the lifetime staff discount. Employees who had worked for the retailer for at least a decade had previously received the 20% discount, but on 1 July M&S lengthened the service period to 25 years.

Machin said he had decided to make a change after discovering that 50,000 people who had left M&S were able to claim the discount, which he said was hitting profits.