One shopper said the difference between same-size garments was "staggering".

When asked about the issue by the BBC, H&M's boss said: "There's always improvement to make."

Helena Helmersson insisted that the retailer had "lower returns than many others".

But she said the firm was taking proactive steps to ensure that "whatever customers buy, they want to keep", for instance by improving H&M's size guides for customers.

Under its new fee, H&M customers must pay £1.99 to return items bought online, with the cost taken from their refund. Previously all returns were free.

The news prompted many shoppers to vent their frustration about inconsistent sizing online and on social media.

"Returns are not always the customer's fault," said another. "When manufacturers tighten up their quality control and standardise sizes then we'll have fewer returns!"

Concern about inconsistent sizing appears to go wider than just H&M, although some shoppers said the retailer had a particularly bad track record.

"Among big retailers H&M is particularly bad at sizing. Their M can be anything from XS to XXL," one shopper told the BBC.

Another urged H&M to open more High Street shops with changing rooms, so that shoppers can try on items in person.

After buying a pair of green H&M trousers a month ago, last week Lesley Hodgson, 38, from Halifax, liked them so much that she ordered two more pairs in the same size in different colours.

So she was shocked to find that when they arrived, the new black pair fit her perfectly, just like the green - but thebeige pair were too small, despite all of them being labelled a European size 40.

"I have to really breathe in to fasten them, they're far too tight everywhere," she told the BBC.

Ms Hodgson said she had contacted H&M as she didn't think it was fair she had to pay the returns fee.

"It's not like I've ordered multiple sizes to see which one fits me. I already knew they fit me, I just wanted more colours, so I bought them in good faith," she said.

An H&M spokesperson said all customer queries are dealt with through the firm's dedicated customer service team.

Catherine Shuttleworth, a retail expert, said inconsistent sizing is "a real shopper bug bear".

"One of the biggest reasons to return clothes is poor fit," she told the BBC. "So it's commercially sensible for clothing retailers to work on the communicating of sizing."

Ms Shuttleworth said one way retailers could do that was by using online tools, powered by artificial intelligence, which asked customers questions about their height, weight, age, gender and body shape to help find the perfect clothing fit.

Rival retailers such as Zara, Boohoo, Uniqlo and Next already charge for online returns.

On Wednesday, H&M's head of investor relations, Joseph Ahlberg, said the retailer had introduced the fee "to remind consumers to be mindful of making returns due to the climate impact".

Speaking on a conference call with the media and investors, Mr Ahlberg said: "It's a win for the consumer, helping them to make good choices, it's a win for the environment, with less transport and emissions, and it also will benefit our model I think over time.

"We have an omni-model and this is part of the advantage of offering our fashion both in stores and online."

Demand for online shopping has risen sharply since the pandemic, but it has also led to a big increase in the number of items being returned because they do not fit, or are not as expected.

Traditionally big retailers have offered free returns for online purchases to attract customers, but they can be costly due to the work involved in processing returned stock.

Many shoppers are also becoming more aware of the environmental impact of deliveries and returns. Fewer postal returns means fewer delivery vehicles travelling up and down with parcels.

Additional reporting by Emma Pengelly.

Your rights when it comes to returns

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