The letter from PAN policy officer Nick Mole says: "I am writing to you on behalf of the Pesticide Action Network UK (PAN UK) to ask about your position regarding the sale of home and garden products that contain the active substance glyphosate, and to urge you to withdraw products that contain it from sale."
Glyphosate is the active ingredient in 223 products available in the UK.
In its letter PAN refers to the report by the WHO cancer agency, the IARC, which assessed glyphosate as a "probable carcinogen" - also the decision by German retail group Rewe and Swiss companies COOP and Migros to stop selling glyphosate products.
Roundup Consumer business director Gary Philpotts said retailers should continue selling products with the ingredient because it is safe to use and approved by member states.
See more on this story in Horticulture Week 24 July.