Studies of a random sample of more than 600 farmers, who had done an average of 54 days of sheep dipping, found that 19 per cent showed nerve damage symptoms, says the report from the Institute of Occupational Medicine in Edinburgh.
The document provides the first clear evidence that long-term low-level exposure to the chemicals, used for combating problems such as blowfly infestation and sheep scab, can damage the nervous system. High dosages have long been known to be dangerous. Exposure from concentrated dip was most likely to cause nerve damage, but splashes from the diluted product were also a health threat.
The results were disclosed in a written parliamentary answer by Jeff Rooker, the Farming and Food Safety minister, who said the study would be "urgently" considered in committee to see whether new advice or measures were necessary for safe handling of OP sheep dip. Mr Rooker said he had written to dip manufacturers seeking a meeting "within the week" to discuss what action they proposed to take.
But last night the principal campaigner against the chemicals, Elizabeth Sigmund, who runs the OP Information Network, described the failure to introduce an immediate ban on the use of OP pesticides as "deeply irresponsible".
She said: "Urgent action is now necessary. To wait and hide behind the chemical companies and yet another committee is not only cowardly but also deeply irresponsible. This is prevarication of the most blatant kind and on a serious issue of ... health concerning all British sheep farmers."
The OP Information Network says it has on its database 740 sheep dippers who claim to have suffered ill-effects from thedips. OPs are related to nerve gases used in chemical warfare and their neurological effects are said to be similar to those allegedly suffered by soldiers claiming they have Gulf War syndrome.
From the 1970s, the government insisted on the use of OPs to dip flocks, but since 1992 their use has been optional. Farmers say the chemicals have caused them to suffer chronic fatigue, memory loss and aching limbs. Some say they are suicidal.
"There are around 70,000 sheep farmers in the UK and we are looking at one in five suffering the effects of OPs," said Peter Beaumont, development director of the Pesticides Trust. "That's a lot of people who have got problems with their nervous system. These are highly dangerous chemicals."
The Government has told farmers they must use up, within the next two years, supplies of the main organophosphate, Diazionon, which is used on crops. "We want to see a ban on the use of all OPs and the introduction of a no-fault compensation scheme. Clearly ... a lot of people deserve compensation," Mr Beaumont said.