Gardeners across the UK have been warned not to eat home-grown vegetables contaminated by a powerful new herbicide.

The Royal Horticultural Society has been flooded by calls from gardeners who have seen potatoes, beans, peas, carrots and salad vegetables wither or become grossly deformed. The affected gardens were contaminated by manure originating from farms where the hormone-based herbicide aminopyralid has been sprayed on fields.

Aminopyralid, which is found in several products produced by Dow AgroSciences, is not licensed to be used on food crops and carries a label warning farmers not to sell manure that might contain residue to gardeners.

The contamination probably came from treated grass was made into silage, then fed to cattle during the winter months. The herbicide remained present in the silage, passed through the animal and into manure that was later sold. Horses fed on hay that had been treated could also be a channel.