Government scientists identified the cause of the problem as a new weedkiller being used by farmers on fields two years ago.

The substance, containing aminopyralid, was sprayed on grass which was eaten by cows and then transferred into manure used by gardeners and farmers alike.

And the manure, which has caused problems across the country, has now devastated a third of all crops at Green Lane allotments.

Susan Garrett, 57, is the chairwoman of the Green Lane Allotment Association.

She said: “As the manure decays it releases this herbicide which stays in the soil for around four weeks, during which time it affects most vegetable crops.

“The problem has worked all the way down the chain of supply until it gets to the gardener.”

At first the government’s Pesticides Safety Directorate advised against eating the crops but last Friday it announced they were safe to eat.

Mrs Garret said: “The plots have been affected in varying degrees depending on how the manure has been spread.

“Some have shrivelled or deformed and some have failed to grow at all.

“We have one family with two little girls and their plot was very badly affected. It nearly made them give up on their allotment because if someone advises you not to eat something you don’t want to risk it.”

Some allotment holders at the site, just off Green Lane, have started their own compost piles to ensure their fertiliser is safe for next year.

For the latest updates, log onto Green Lane Allotment Association website at www.glal lotments.btik.com