Less than a third (29 per cent) of bras sold in an undercover investigation were rated as a fit by experts.
Consumer champion Which? sent 11 mystery shoppers to buy underwear with the help of staff during 70 visits.
The volunteers, aged 25 to 75, were a DD cup-size or bigger.
Around a quarter (18) of the 70 fittings were given scores of zero for the study and no store's service was deemed good enough to recommend.
Experts suggested the customers who experienced the worst fittings would have been better off "picking something off a market stall".
Some of the women were sold different sizes by different shops, the survey found.
In one case this varied by seven sizes from a 34FF to 40D, while another woman was sold the same bra in two House of Fraser branches but in sizes 34C and 34F – neither of which were a good fit, Which? said.
Badly fitted bras can cause neck, shoulder and back ache and poor posture, as well as affecting appearance.
Consultant bra technologist Gillian Spencer and lecturer and technical designer Laura Savery assessed the fit of bras sold to the researchers.
They looked at the fit and comfort of cups, wire and straps.
David Morris, principal lecturer in contour design at De Montfort University, who was also on the panel, said: "A good fitter will choose the right brand and style or wire shape for the individual's body, and check fit and comfort.
"A larger bust will generally need a firmer fit, more supportive back elastic and a more reinforced fabric.
"A well-fitted bra can make a big difference to your profile and how you feel."
Of the six retailers tested, John Lewis and Bravissimo received the best overall scores, with 45 per cent and 43 per cent for fittings.
Bottom of the list – which also included Debenhams, la Senza and Marks & Spencer – was House of Fraser, which was given a 28 per cent score.
Jenny Driscoll, from Which?, said: "Whatever their bra size, women want to look good.
"Heading to the high street for a fitting might seem like a simple solution, but the results we found were shocking – one bra was so poorly fitted there was room for a pair of socks in the cups.
"If stores are going to offer this service to customers they need to up their game: do it properly or don't do it at all."
Annette Warburton, head of lingerie buying for Debenhams said: "We pride ourselves on providing first-class service throughout our stores, which includes our bra fitting appointments.
"Our research has shown that the vast majority of customers are happy with the Debenhams' bra fitting service and we work hard to make sure we provide a welcoming, professional service.
"We have expert bra fitters in every store. Our bra fit training schedule for 2010 is now in full flow with product knowledge training, best practice in customer service and specialist training in aspects of lingerie fitting including fuller bust, mastectomy and maternity.
"We welcome the feedback from Which? and will be incorporating this into our training programme."
Which? bra fitting scores in percentages:
1 John Lewis 45%
2 Bravissimo 43%
3 M&S 36%
4 Debenhams 35%
5 La Senza 31%
6 House of Fraser 28%