Since allotment holders in the Wakefield area drew attention to the problem, the Royal Horticultural Society and the government's Pesticides Safety Directorate have issued warnings.

The alert is mainly about a substance called aminopryalid in an agricultural herbicide called Forefront - although similar substances may be causing similar effects.

It is a new kind of "hormonal herbicide" – meaning it interferes with the growth signals of broad-leaved plants like dandelions and docks, so they shoot up too fast and starve, while the surrounding grass is unaffected.

It has been declared safe to graze cows on treated grass, or on hay and silage made from treated meadows. The herbicide passes through their systems more or less undigested. But that means it is present in their manure and gets into their bedding straw too.

The packaging of Forefront includes warnings about not using affected manure and straw on vulnerable plants –including tomatoes, potatoes, peas, beans, carrots, lettuce, delphiniums, phlox and roses. But the warning is not always passed on.

Forefront was launched three years ago and Britain was the first European country to use it. Last year, some professional potato growers lost crops after spreading manure. An emergency education campaign, aimed at livestock farmers, vegetable growers and spraying and spreading contractors solved that problem.

But since last Christmas, the Royal Horticultural Society has been getting an average of 20 calls a week from amateur gardeners reporting similar symptoms in plant foliage.

About 20 people at the Green Lane Allotments in Horbury, near Wakefield, joined the complainants a month ago. They all bought manure from the same farmer.

The chairman of the allotments association, Susan Garrett, believes the farmer had no reason to think the manure was poisonous. The pesticides authority was informed, but its advice to gardeners is to make their own inquiries and pass the results to the manufacturers, Dow AgroScience.

The authority says the weedkiller does break down and should not persist in the soil for more than a year. But it can last longer in a compost heap. Confirming its presence is "costly". And "if in doubt, it is probably best not to consume affected crops".

A Dow spokesman said they had to give the same advice but the risk to human health appeared to be negligible.

He said: "It's not the first case of herbicide residues in manure and it may not be the only one that is current. Forefront is being blamed for all sorts because the name has got into discussions on the Internet. There are products that act in a similar way and there are plant diseases which look similar. However, we accept there is a problem to address."

Mrs Garrett said: "It's been very upsetting to see so much hard work and money go to waste, especially as one reason for having an allotment is to try to get away from agri-chemicals. I hope that at least our experience helps somebody else to avoid the same."

The pesticides safety authority said last night: "We do not consider that residues of aminopyralid in manure present a risk to consumers. However, we will be seeking samples for analysis, in order to confirm the presence of aminopyralid and determine whether further regulatory measures are necessary."