Local councils now use more pesticides than any other group except farmers. They spray over 600 tons of active ingredients every year - including paraquat, 24D (one of the constituents of Agent Orange, used to defoliate Vietnam) and other highly dangerous chemicals.
Government figures show that the spraying has risen by 13 per cent in recent years.
The Pesticides Trust, which monitors use of the chemicals, points out that unlike farmers local authorities spray the chemicals "in close proximity to the large numbers of people and in the environments which the public regularly uses".
It adds: "This means that a great many people actually see and potentially feel the effects of local authority-applied pesticides." The more polished and weed-free a park appeared, it said, the more likely it was to have been heavily sprayed.
In a speech to the Institute of Leisure and Amenity Management's annual conference in Bournemouth tomorrow, Mark Davis, the Trust's national projects officer will say: "We are particularly concerned about the exposure of people to pesticides in parks.
"Children play there and roll around on the grass which has been sprayed, and people walk their dogs there." He will add that the "fall out from this chemical warfare" ends up in human water supplies and in the food chain, killing songbirds.
The criticism comes at a particularly sensitive time. Last week the Government published what it described as its "most thorough review ever" of the effects of one kind of pesticide - organophosphates. And tomorrow environment minister Michael Meacher meets anti-pesticides campaigners and doctors to hear their case. Children are particularly at risk, says the trust, because safety levels for pesticides are calculated for adults and do not take into account children's greater sensitivity and smaller bodies.
An alarming study in the United States three years ago suggested that children living in houses with gardens treated with chemicals were four times more likely to contract soft tissue cancers than youngsters from pesticide-free homes.
Another study, by the prestigious US National Cancer Institute showed that dogs living in houses with lawns treated with 24D were twice as likely to develop cancer as others.
In Britain a local authority was prosecuted after a child suffered allergic reactions when accidentally sprayed with glyphosate - a relatively safe herbicide used on grass before white lines for sports fields are laid down.
Liz Sigmund, of the Organophosphate Information Network, says she has been contacted by local authority workers whose health has been affected by the pesticides.
Jonathan Curtoys, Pesticide Policy Adviser to the Royal Society for the Protection of Birds, says that using pesticides in parks is likely to endanger song thrushes, robins, blackbirds, warblers and other birds.
The Pesticides Trust and the Institute for Leisure and Amenity Management are now awarding special "green flags" to parks that reduce their use of pesticides and improve safety in other ways. Last year only seven parks in Britain got the award - including Battersea Park and Highgate Woods in London - and the scheme is now short of money.
The Government report, by a group of officials from different Whitehall departments, concluded that there is "no medical consensus" on the effects of organophosphate pesticides and so there was no "scientific or objective basis" for a register of people claiming to suffer damage to their health.