The Soil Association and the Organic Growers Alliance (OGA) warned growers to take more care when sourcing manure. Symptoms of aminopyralid contamination include changes in the shape of leaves and fruit, reduced yield and seed germination and even death of young plants it said.
The herbicide is consumed by animals before being used as manure and remains active, damaging crops and can even pose a threat to humans, said the Soil Association.
Soil Association horticultural advisor Ben Raskin said:"We urge growers to be careful about where they are sourcing their manure from. This product should never have been relicenced. Manure contaminated with this herbicide is no longer a valuable resource for gardeners but a toxic waste product.
"The fact that this hormone weed killer is still causing significant problems for growers despite industry attempts to prevent damage means that it should be immediately banned by the government."