Officially the BSE outbreak was caused by the feeding of meat and bonemeal to cattle. Farmer Mark Purdey believes differently, and has been crusading for some time to tell his version of the truth, which he backs up with a number of scientific claims. He told visitors to London's Organic Food and Wine Festival about his quest.

Mr Purdey wholly refutes the official explanation, claiming that other countries which have fed meat and bonemeal to their herds, such as those in the Middle East, have not had cases of BSE. In addition, some cattle on a MAFF-sponsored grass-only farm in the UK did develop the disease, he said.

Instead, he cites the use of organophosphates in farming as being at the root of the problem. Farmers were ordered to use the organophosphate Phosmet in 1984 to eradicate the warble fly, a few years before BSE was first detected. Mr Purdey claimed that organophosphates are a dangerous chemical group which cause the release of a number of free radicals in the blood stream. The have also been linked with various illnesses.

Initial tests of Mr Purdey's theory failed to back it up and his claims were dismissed. He claimed that the procedures were flawed and mounted a second raft of testing, which showed some of the biological characteristics but not all of those he hoped to observe. He therefore set out to prove that the presence of organophosphates was crucial to the development of the disease, if not its sole cause.

Mr Purdey travelled the globe to see if he could find similarities in areas where there were clusters of BSE or CJD. He discovered identical mineral conditions in places such as Colorado, Iceland and Slovakia where the disease was present, namely high levels of manganese and low levels of copper, aluminium and zinc. He then noticed that all the areas were exposed to high levels of UV radiation, concluding that the UV was acting as an oxidising agent. He therefore surmised that the presence of high manganese and low amounts of the other metals, which act as anti-oxidising agents, was responsible for the outbreaks. For UV in other parts of the world, he substituted organophosphates in the UK as the oxidising agent.

Much work needs to be done on Mr Purdey's theories and much of it is highly technical. Nevertheless, his conviction means that the BSE debate is set to continue.

By Hugh Westbrook, correspondent