Bra sizes were developed from blouses based on men's proportions, experts say (file image)

You've been professionally measured and spent a fortune on new bras – but they still don’t fit.

Now, experts believe they know the reason why.

Surprising as it may seem, the sizing system used by the lingerie industry dates back a hundred years – and is based on men’s measurements.

Sue McDonald, a bra fitting specialist who has researched the system’s origins, said bra sizes were developed from blouse sizes, which were in turn based on male military uniforms from World War I.

Critics say the measurements are not suitable for women today. Atul Khanna, a consultant plastic surgeon, said that while women back then were typically slimmer and may have had boyish figures, the female shape today tends to be much fuller.

Some 60 per cent of women in the UK are now overweight or obese – meaning millions will struggle to find a bra that fits them properly.

Joanna Scurr, an expert in breast health from Portsmouth University who carried out separate research into bra measuring, said that the cup-sizing system was never designed to go above a size D.

‘Our research shows that as breasts get larger, the measuring system gets less accurate,’ she said.

‘Bra-fitting becomes more of an issue as the breasts and the body get larger. We know that within the UK and in many other parts of the world, women are getting bigger and we are seeing an increase in breast size as well.

‘I’ve seen reports of ZZZ-sized females and the system was never designed for that.’

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Mr Khanna, who performs breast-reduction operations on women who are in so much pain that surgery is the only option, asked a group of overweight women if they suffered from common symptoms of an ill-fitting bra such as back and neck pain.

Nine in ten said their bra was painful – even though most of them had been professionally fitted. Rashes were the most common problem, with two in five saying their bra chafed.

Some 35 per cent had sore shoulders, 20 per cent had neck pain and a similar number suffered back-ache. Mr Khanna, who is a member of the British Association of Aesthetic Plastic Surgeons, also found that a bra that was the wrong size according to the traditional system was often a better fit than one that was supposedly the right size.

Bra sizes are based on blouses developed from WWI uniforms, similar to those portrayed above, say experts

But it is not just overweight women who run into difficulty – for slim women with large chests are also likely to find lingerie shopping tricky.

This is because a quirk in the sizing system means that they will be pointed towards bras that are very small around the back, which makes them extremely tight.

The current system involves taking two measurements – one under the bust and one over it. The under bust number gives the back size, such as 32 or 34, while the difference between that and the measurement over the bust gives the cup size, such as C or D.