A High Court judge ruled the government had failed to comply with its obligations under a European directive to protect rural residents from possible harmful exposure to toxic chemicals during crop spraying.
It follows a seven-year battle by Georgina Downs, who was yesterday granted a judicial review of the policy on pesticide safety and use. She said she was "very pleased" by the judge's decision over what she described as "one of the biggest public health scandals of our time".
The Department for Environment, Food and Rural Affairs (Defra) said it would now examine the judgment in detail.
Miss Downs, 35, who lives with her parents on the edge of farm fields near Chichester, West Sussex, formed her independent group, UK Pesticides Campaign, in 2001.
Mr Justice Collins, sitting in London, said Miss Downs was 11 when she was first exposed to pesticide spraying and "began to suffer from ill-health, in particular flu-like symptoms, sore throat, blistering and other problems".
Miss Downs said in court that the government failed to cater for the needs of residents such as herself "who are repeatedly exposed to mixtures of pesticides and other chemicals throughout every year, and in many cases, like mine, for decades".
She said residents were not given prior notification of what was to be sprayed near their homes and gardens.
Miss Downs gathered testimony from rural residents reporting problems such as cancer, Parkinson's disease and asthma, which they believed could be linked to crop-spraying.
The judge said she had produced "solid evidence" residents had suffered harm, and that the Environment Secretary, Hilary Benn, must rethink the way spraying was controlled and the risks to human health assessed.
The judge said "defects" in Defra's approach to pesticide safety "contravene the requirement" of a 1991 EC directive that harmonises the regulation of "plant protection products".
He said Mr Benn "must think about and consider what needs to be done", adding: "He must take steps to produce an adequate assessment of the risks to residents."
Miss Downs said it was a "very significant landmark ruling".
During the hearing, Defra had argued its approach to the regulation and control of pesticides was "reasonable, logical and lawful in all the circumstances".
Yesterday, it said it would look at the ruling to see if there were ways it could strengthen its system.
The ruling applies only to England and Wales, but a Scottish Government spokesman said: "There are good reasons why Scotland would want to remain in line with the rest of the UK. We will look carefully at the implications of this judgment."