IF you think you have kept the same slim waistline for the past 10 years, prepare to be disappointed.

Britain’s high-street shops have been flattering the vanity of their customers by understating the true sizes of their trousers on the label by an average of 3in.

Research by The Sunday Times in leading clothes chains has found the biggest discrepancy to be on a pair of men’s jeans sold by French Connection, where the actual size exceeded the stated measurement by 6in.

Other brands whose trousers were bigger than the stated size included Gap, H&M and Zara.

The practice, known as “vanity sizing”, began with women’s clothing in America. It is now used in men’s trousers and has spread to Britain.

Fashion experts said they were surprised by the findings.

Jeff Banks, the designer, said a discrepancy of an inch or two might be justified by styling differences or variability in manufacturing. Anything more, he said, looked like an attempt to make customers think they were thinner than they were.

“It’s deluding customers,” he said. “Changing things by one size may be sufficient but to do it by this much is something the consumer does not like.”

Customers buying trousers last week said they had assumed the size on the label was the size of waist the trousers were meant to fit. They were disappointed to find they were fatter than they had believed.

Lisa Hodgson, 23, an actress from Surrey Quays, south London, bought a pair of 25in jeans at the Topshop concession in Selfridges in the capital and was surprised, when given a tape measure, to find she was more than 27in around the waist.

“I can’t believe it,” said Hodgson. “Topshop have figured out how to keep me coming back.”

Of the shops where the research was carried out, French Connection proved to have understated the waist size of their jeans by the greatest margin.

On one range of men’s slim-fit jeans, a size 30in measured 36in, and a size 32in was 37.5in.

French Connection bootflare jeans for women understated the waist size by as much as 4in. A size eight pair, which should have a waistline of 26in, was actually 30in. However, a size 12, which usually has a waistline of 30in, was only 2in bigger.

Jeans at Zara, the Spanish-based high-street retailer, were usually 4in bigger than was claimed on the label. A pair of Zara regular-fit jeans for men at size 32in waist was in fact 36in.

With women’s clothing the discrepancy in Zara’s jeans was narrower. A pair of dark blue jeans in size 12, which usually has a waistline of 30in, measured 31in. A pair of size 16 jeans, which should be 34in, were exactly that measurement.

Other high-street stores that appeared to be practising vanity sizing included H&M and Gap, the US-based retailer.

The discrepancies for designer brands such as Gucci and Calvin Klein were smaller and for some products the sizes were exactly the same as on the label.

In America, vanity sizing is widespread. A Sunday Times reporter checked sizes in Washington and found that at Gap and French Connection, they were understated by 3in-4in.

This weekend the British high-street retailers all denied trying to mislead customers.

French Connection said: “We will look into the claims. We don’t practise vanity sizing.”

Gap said the discrepancies might be due to the different shapes of their ranges.

Zara and H&M said the actual size of the garments should be virtually the same as that stated on the label.

H&M said: “The labels on our garments are the actual sizes. H&M do not practise vanity sizing. Our quality-control department and buying department are looking into this issue.”

Additional reporting: Chloe Lambert, Jon Swaine and Imogen Morizet in Washington

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