General Mills would not specify the timing of the increases, but said it had informed trade customers in early June.
Trade customers can register and preview ranges before placing an online order, saving time posting out brochures and marketing material.
Another contender, Liz Kendall, said: "The Sunday Trading Act works: retailers can trade, customers can shop, shopworkers can spend time with their families.
A spokesman for the company told Money: "Howdens' pricing is based on the confidential trade discount that it gives individually to each of its 430,000 trade customers.
"We support the current arrangements, which work well and mean retailers can trade, customers can shop, and shopworkers can spend time with their families," she said.
When designers need utensils or materials, their requirements tend to be specific, making it invaluable for them to work near a large hardware store that stocks an extensive range of products for trade customers.
He said Kingfisher also served the DFM customer with its continuing expansion of Screwfix – which serves trade customers – and the launch of a new Homefit service, providing customers access to vetted local tradespeople at pre-agreed prices.
The trade customers have been and gone, leaving just a few members of the public to wander round the stalls, as traders begin to sweep up the flotsam and jetsam of another busy morning.
