Marks & Spencer has defended a policy of charging extra on some of its bras that are bigger than a size DD.
The High Street retailer said that the added cost - typically £2 - was "standard industry practice".
M&S added it found most customers "were happy to pay a small premium for the specialist work" needed to make larger sizes of their bras.
But the policy has drawn protests with 900 people joining a Busts 4 Justice protest group on the Facebook website.
Its founders argue it is unfair that smaller bras are cheaper, because this logic does not apply to other clothes.
Beckie Williams, 25, from Brighton, said she set up the social networking site group after an "unsatisfactory" correspondence with the retailer.
She said M&S replied to her letter claiming the extra material needed lay behind the higher price at the tills.
If you don't charge a size 20 woman more for a pair of trousers, then why should you pay more for an E cup bra?
"That's fair enough," Ms Williams told BBC Radio Five Live. "But they don't apply the same policy to other clothes that use more material and more resources to make as well.
"I just think it should be one policy for all items of clothing."
The M&S website advertises the Ceriso Polka Dot Padded Bra for £12, rising to £14 for the version for DD to G sizes.
There are similar differences in price for other ranges, though many of the store's more basic designs cost the same, no matter the size.
M&S said it offered "the most comprehensive DD+ collection and one of the most competitively priced on the High Street" and offered the same choice of colour, styles and shapes for all customers "no matter what size they take".
It also said the majority of its customers had told it they were happy to pay a small premium for the specialist work on larger size bras.
"Our bras are much less expensive than many other specialist larger cup size lingerie retailers, where they could cost around £60 or more," the firm added.
Ms Williams said that £2 was not a huge sum to pay but that it was still "an unfair tax".
"This is not something that we have chosen," she said.
"And I have been a size 8 all my adult life, but have never benefited from smaller priced pants.
"If you don't charge a size 20 woman more for a pair of trousers, then why should you pay more for an E cup bra?"
Most of the those posting comments on the Busts 4 Justice site are supportive of the women's efforts.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.