From the related BRC press release (I can't find the letter itself):
While UK retailers have been very successful in insulating consumers from the cost of rising business rates and labour, the recent devaluation of the pound in relation to our most important trading currencies is compounding economic headwinds, while years of deflation have left little margin to absorb added cost from import tariffs and administrative burdens.
There is of course a huge cushion to absorb these extra costs, it is called “rent”. As long as rents go down in line with the extra costs, retailers in general will be just fine (successful tenants will replace inefficient owner-occupiers). But that's not the weird part:
Moreover, failure to strike a good Brexit deal by 2019 would have a disproportionately severe impact on retailers and their customers, because if the UK fell back on to World Trade Organisation rules the new tariff rates that the UK would apply to imports from the EU would be highest for consumer staples like food and clothing.
For example, the average duty on meat imports could be as high as 27%, while clothing and footwear would attract tariffs of 11-16% versus the current zero-rating for all EU imports.
Falling back on to WTO rules would also increase the cost of sourcing from beyond the EU. The import cost of women’s clothing from Bangladesh would be 12% higher, while Chilean wine would be 14% dearer for importers. This contrasts with duty rates that would apply to raw materials and semi-finished products, many of which would be zero-rated or attract rates of duty of below 10%.
Hang about, I thought that the WTO has a system of maximum import tariffs which a country can impose (subject to loads of silly exceptions), not minimum tariffs?
They way they say it, the UK would have to impose higher tariffs on certain things. Can this possibly be correct? Can't WTO members just unilaterally abandon import tariffs?