Gardeners have been warned not to eat homegrown vegetables contaminated by a powerful new herbicide.

The Royal Horticultural Society (RHS) has been inundated with calls from gardeners who have seen their fruit and vegetables grow deformed.

One allotment holder said he fears he will have to burn his crops after they showed signs of disease.

The problem began when farmers sprayed their crops with aminopyralid weedkiller.

The chemical dissipates into the soil and binds to grass and hay, which is then eaten by livestock. Manure from the livestock is used to make compost, which has been used on allotments.

Jacob Nowinski, of the Whitehawk Community Food Project, based in Whitehawk Hill Road, Whitehawk, Brighton, said some of the group's allotments had been contaminated.

The allotments are used by schoolchildren and members of the public, who are taught by experienced gardeners to grow healthy, organic food.

Mr Nowinski said: "We have 12 allotments and most of them are used for fruit and veg. About 10% are showing signs of damage.

"We have got our compost from local stables for the past five years. We've never noticed anything unusual before. "You cannot eat diseased plants. It will be devastating to have to burn the crops after children have worked so hard to grow them.

"If this is just one herbicide which has ended up in the food chain, what is to say many more herbicides haven't as well?" Guy Barter, head of horticultural advisory services at the RHS, said: "Our advice is not to eat the vegetables because no one seems to have any idea whether it is safe to eat them and we can't give assurances.

"Until recently weed killer damage was fairly unusual and usually due to mistakes in applying lawn weedkillers and using contaminated watering cans and sprayers.

"This year not only are there far more instances reported to us than usual, but farmyard manure is frequently implicated as the source of the weed killer."

Gardeners who use compost from other animal sources have not seen their crops contaminated.