Gardeners have been warned not to eat home-grown vegetables contaminated by a powerful new herbicide that is destroying gardens and allotments across the UK.

The Royal Horticultural Society (RHS) has been inundated with calls from concerned gardeners who have seen potatoes, beans, peas, carrots and salad vegetables wither or become grossly deformed. The society admitted that it had no idea of the extent of the problem, but said it appeared 'significant'. The affected gardens and allotments have been contaminated by manure originating from farms where the hormone-based herbicide aminopyralid has been sprayed on fields.

Dow AgroSciences, which manufactures aminopyralid, has posted advice to allotment holders and gardeners on its website. Colin Bowers, Dow's UK grassland marketing manager, told The Observer that links to their products had been proved in some of the cases, but it was not clear whether aminopyralid was responsible for all of them and tests were continuing. 'It is undoubtedly a problem,' he said, 'and I have got full sympathy for everyone who is involved with this.'

He said the company was unable to advise gardeners that it was 'safe' to consume vegetables that had come into contact with the manure because of pesticide regulations. 'All we can say is that the trace levels of aminopyralid that are likely to be in these crops are of such low levels that they are unlikely to cause a problem to human health.'

The Dow website says: 'As a general rule, we suggest damaged produce (however this is caused) should not be consumed.' Those who have already used contaminated manure are advised not to replant on the affected soil for at least a year.

Aminopyralid, which is found in several Dow products, the most popular being Forefront, a herbicide, is not licensed to be used on food crops and carries a label warning farmers using it not to sell manure that might contain residue to gardeners. The Pesticides Safety Directorate, which has issued a regulatory update on the weedkiller, is taking samples from affected plants for testing.

Problems with the herbicide emerged late last year, when some commercial potato growers reported damaged crops. In response, Dow launched a campaign within the agriculture industry to ensure that farmers were aware of how the products should be used. Nevertheless, the herbicide has now entered the food chain. Those affected are demanding an investigation and a ban on the product. They say they have been given no definitive answer as to whether other produce on their gardens and allotments is safe to eat.

It appears that the contamination came from grass treated 12 months ago. Experts say the grass was probably 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.

Bryn Pugh, legal consultant at the National Society of Allotments and Leisure Gardeners, said he was preparing claims for some members to seek financial compensation from the manure suppliers. But it was extremely difficult to trace the exact origins of each contaminated batch. 'It seems to be everywhere. From what I know, it is endemic throughout England and Wales. We will be pressing the government to ban this product,' he said.

Aminopyralid is popular with farmers, who spray it on grassland because it controls weeds such as docks, thistles and nettles without affecting the grass around them. It binds itself to the woody tissue in the grass and only breaks down when exposed to bacteria in the soil.

Shirley Murray, 53, a retired management consultant with an allotment near Bushy Park in Hampton, south-west London, said several of her allotment neighbours had used the same manure bought from a stables and all were affected. 'I am absolutely incensed at what has happened and find it scandalous that a weedkiller sprayed more than one year ago, that has passed through an animal's gut, was kicked around on a stable floor, stored in a muck heap in a field, then on an allotment site and was finally dug into or mulched on to beds last winter is still killing "sensitive" crops and will continue to do so for the next year,' she said.

'It's very toxic, it shouldn't get into the food chain. You try to be as organic as you can and we have poisoned ourfood. I've been everywhere, emailed all the right people, but nobody will speak on the record to guarantee what is safe to eat. We all think it is a scandal. Not to mention what it has cost in time and money.'

Pesticide expert Professor Vyvyan Howard, a toxico-pathologist at Ulster University, said it was 'a very powerful herbicide' but in his opinion was 'unlikely to pose any human health risks'. However, advice about its use should be strengthened, he said. 'I think the thing that is going to drive this is the commercial damage that could be done to market gardeners,' he said.

Guy Barter, the RHS head of horticultural advisory services, said they were receiving more than 20 calls a week. '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 any assurances,' he said. 'It is happening all over the country. A lot of cases we are seeing is where people have got manure from stables and the stable have bought their hay from a merchant, and the merchant might have bought hay from many farmers, possibly from different parts of the country. So they have no idea where the hay came from. So finding someone to blame is quite difficult.' Weedkiller in the soil should dissipate by next year, but in stacks of contaminated manure it might take two or more years to decay, he added.

Dow is planning a major publicity campaign to reiterate warnings to farmers over usage, and to encourage allotment holders to check the provenance of manure that they put down in an effort to prevent the problem escalating. On compensation, it was less forthcoming. 'There is no easy answer to that,' said Bowers. 'The first port of call is always where the manure comes from. From that point on, I can't really comment.

'The chain is horrendously complicated. In the cases we have managed to trace back, we might find that the farmer who supplied the manure didn't spray anything himself, but he might have bought in a couple of bales of silage from one of his neighbours, and that farm might have sprayed.'

Robin and Christina Jones spread a large amount of manure over their flower garden and vegetable patch at their home in Banstead, Surrey. When the potatoes failed, Robin took a sample to the RHS, which identified aminopyralid. His neighbour, who bought from the same source, suffered the same problems. 'We have lost 80 per cent of our vegetable patch,' said Jones, 65, a retired sound engineer. Raspberries, French beans, onions, leeks, even a newly planted robina tree were all affected. 'We are distraught. But what worries me is that the courgettes look very healthy. Had we not had the problem with the potatoes, we might never have realised. Now we are advised not to eat them.

'This is a very serious issue, and people must be made aware of the advice not to eat vegetables grown in contaminated manure.'

Sue Ainsworth, 58, an education consultant, said around 20 allotments at her site in Hale, Cheshire, had been affected. 'We first noticed with the potatoes. As they came through, they were deformed, all curled over and rotten underneath. But the worry is that the courgettes also planted on the manure are fine - but are they safe to eat? This must have affected thousands of people. I am really worried about this product and really think it should be withdrawn.'

She said the farmer who supplied the manure said he had used nothing unusual. 'But he may have bought in the straw and genuinely knew nothing about the herbicide used.'

Susan Garrett, 57, an IT consultant, said 20 plots were affected at her allotment in Wakefield, West Yorkshire. 'And that is just the plants we can see are damaged. We are angry it has been allowed to happen - not with the chemical company, but because there doesn't seem to be any protection for us or anything to stop it happening again.'

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How to deal with the problem

Do you have contaminated manure?

Tell-tale symptoms of crop damage include distorted foliage, with cupping of leaves and fern-like growth. There are no remedies once damage has occurred. Susceptible crops include potatoes, tomatoes, beans, peas, carrots and lettuce.

How should you deal with the affected area?

Experts say rotavation is the best practice, or forking over several times as soon as possible. This incorporates the plant tissue into the soil, where it will decompose and the chemicals will eventually be degraded by soil microbes. Repeat the rotavation in late summer/early autumn.

Should you replant this season?

No. The plant residues need to be given time to break down. The advice is not to replant for a year.

Why has the chemical lasted so long?

Aminopyralid, like other herbicides, works by binding strongly to plant tissues. Once the plant's tissues decay, the chemical breaks down in the soil. If manure is stacked it takes far longer.

· This article was amended on Thursday July 17 2008. Its original title referred to a "toxic fertiliser", when we meant "toxic herbicide". This has been corrected.