High street shops have been accused of posing a risk to 'public health' by selling clothes in the same dress size with as much as 8in difference in the waist measurements.
So-called 'vanity sizing' is not only confusing for customers but could also be making women overweight as they do not realise their true size, campaigners say.
While size 10 dresses in Dorothy Perkins and Topshop measure as little as 24 inches at the waist, size 10 skirts from other retailers can go up to as much as 32 inches, a new investigation by journalists has found.
The sizing discrepancy across eight of the UK's most popular women's stores is staggering; in some, a size 10 has a waistline of 32 inches - in fact closer to a size 14 to 16.
Campaigners say vanity sizing - where shops makes their sizes bigger to flatter customers - is causing shoppers to think they are far slimmer than they are, leading them to put off maintaining a healthy weight.
Vanity sizing? In Topshop - traditionally popular with teenage girls - a green midi dress had a 24 inch waist, but this denim jumpsuit in the same size (pictured) was 30 inches
Tam Fry, chairman of the National Obesity Forum, said the issue was doing little to aid public health.
He said: 'Regrettably vanity sizing has been in vogue for some time as manufacturers and retailers fight to keep their customers happy.
'The tragedy is that it works, with millions of women still fooled into believing they are keeping their shape when they are not.
'Statistics show that some 60 per cent are no longer a healthy weight and though the sizing ploy may help their self-esteem in the shop, it does nothing to help their long-term health.'
When measured this week, clothing favourites including Zara, H&M, Miss Selfridge, M&S, Whistles and Topshop showed a huge disparity between their size 10 dresses, trousers and skirts.
A v-neck floral dress from high street favourite H&M. Its size 10 dress was among the smallest with a modest 26in waist - campaigners have called the phenomenon 'vanity sizing'
Across the eight stores, most size 10 items had average waist measurements of between 28 and 30 inches - the equivalent of a European size medium - but some were shockingly out.
In Topshop - traditionally popular with teenage girls - a green midi dress had a 24 inch waist, but a denim jumpsuit in the same size was 30 inches.
At Reiss, many dresses have a 26 inch waist, but trousers and skirts run as large as 30 inches for size 10s.
Meanwhile at Whistles, there was a six-inch difference between size 10s, with dresses coming in at 24 inches and trousers at 30 inches.
And at teen favourite Miss Selfridge, there was a four-inch disparity with some size 10s 26 inches and others 30 inches.
Branding expert Marcel Knobil, founder of Superbrands, said the disparity in sizing across stores has left women frustrated, self-conscious and confused.
'Unfortunately many brands are pandering to customers to make them feel as slimline as possible,' he said.
A size 10 dress from Miss Selfridge with the smallest measurements out of all the garments analysed in the study, with just a 24in waist - some brands went up to 32in
'This leads to different sizes across different shops which can be very confusing to customers.
'The whole issue of size is an extremely sensitive one and brands have encouraged a real doubt and frustration among women.'
Sizes are supposed to be standardised across Europe, but in reality shops have no legal requirement to comply.
A European medium should now come in between 29.3 inches and 31.4 inches - a far cry from even 20 years ago when that waist size would have been considered large.
There is British Standard for women's clothing sizes, created in 1982 by the British Standards Institution, which gives measurements for sizes 8 to 32 - however there are still anomalies among different brands.
According to that, a UK size 10 should have a waist of between 25.1 inches and 26.7 inches.
Research in 2004 found that women's waistlines had ballooned six inches from the last time the nation was measured in 1951.
The Department of Trade and Industry used high-tech body scanners to take 130 measurements from 11,000 people, 50 years after the last full-scale sizing survey.
At Whistles, there was a six-inch difference between size 10s, with dresses coming in at 24 inches and trousers at 30 inches. This size 10 confetti heart frill skirt had a 28in waist
While the average 1950s woman was two inches shorter at 5ft 2in, her chest and hips were 1.5in slimmer at 37 inches and 39 inches.
But the difference in her waist was worrying - in 1951 the average was 27.5, but by 2004 it was 34 inches.
The NHS states that anyone with a waist larger than 31.5 inches should try and lose weight to avoid cancer, diabetes and heart disease, while it labels a 34 inch waist 'very high risk.'
Then, Miss Average was a size 12, but today's female is edging towards a size 16.
A spokeswoman for the British Standards Institution said: 'Many retailers size their clothes according to their customer base, for example if the clientele is older they may be shorter. People prefer to fit into a 12 than a 14 so retailers like to please.'
And she added: 'As body sizes have gotten bigger so have dress sizes.'
Of all the shops tested, H&M had the greatest consistency in sizing, after they vowed to overhaul their measurements last year.
Two dresses and a pair of trousers from the store had an accurate waist measurement of 26' while another pair of trousers was just two inches bigger at 28 inches.
The retailer had previously come under fire from consumers who claimed their sizes were far too small.
As body sizes have gotten bigger so have dress sizes
Last July, they pledged to re-evaluate their sizing guides to better reflect the average woman.
Almost six in ten (57 per cent) English women are now overweight or obese.
Back in June, an investigation by Barclaycard found that British shoppers returned £7 billion of purchases each year, with 40 per cent blaming inaccurate clothing measurements.
A spokeswoman for the London College of Fashion said: 'The problem is that women's sizing really isn't official or uniformed - it's a marketing technique.
'It all varies depending on the brand and the age group they wish to target.'
A spokesman for H&M said: 'We are pleased that the findings of this investigation found H&M sizing to be the most consistent on the high street.
'Following customer feedback, in 2018 we took steps to change our womenswear measurements to be in line with UK sizing - for example the previous measurements and fit of a size 12 are now the measurements of a size 10.
'We hope our customers feel encouraged that we have listened to their valuable feedback.'
All retailers have been contacted for comment.