They said: "Aluminium sulphate poisoning probably led to long-term cerebral impairment in some people in Camelford."
The finding could trigger a rash of new claims for compensation for one of Britain's worst episodes of pollution, in July 1988. Twenty tons of aluminium sulphate was accidentally tipped into a drinking- water reservoir at Camelford in Cornwall serving 20,000 people.
Aluminium is a poison, which can cause brain disease in animals, and humans who have undergone kidney dialysis and been exposed to aluminium in the dialysate solution used for dialysis.
Two years later, 400 people were still suffering symptoms including aches and pains, general malaise and problems with concentration and memory, which they said were linked with the contamination.
Two inquiries failed to prove the aluminium sulphate had caused actual ill effects, although it was accepted the incident and the publicity it triggered caused "mental and physical suffering" in the community. South West Water, which owned the reservoir, was fined pounds 10,000, but a claim for exemplary and aggravated damages was rejected.
The polluted water was discoloured and tasted foul but South West Water, which did not admit the error until 17 days later, said it was safe to drink even for infants, and orange squash should be added to mask the taste.
The second of the inquiries, in 1991 and chaired by Dame Barbara Clayton, concluded the occurrence of brain damage, birth abnormalities and other symptoms after the accident would be expected by chance in any community of that size. It attributed the loss of concentration and poor memory reported by many of those affected to anxiety triggered by the incident, but recommended long-term monitoring of the population.
Today in the British Medical Journal, a team of researchers led by Dr Paul Altmann, a kidney specialist at the John Radcliffe Hospital, Oxford, reject the claim that anxiety was the cause of symptoms. Three years after the poisoning they studied 55 of those affected, and compared them with 15 of their siblings who had not drunk the water.
The team tested affected individuals and the controls to assess their cognitive and psychological function, and found abnormalities in those exposed to the poisoned water. Forty-two of the 55 performed significantly less well than expected on the basis of their IQ.
Paul Tyler, Liberal Democrat MP for Cornwall North, which includes Camelford, said: "There are many people in the Lowermoor area [site of the reservoir] who are still highly aggrieved. We have been calling for a public inquiry and with such a forthright, definitive and expert report on the long-term health problems the Government must now hold one."
The scientists have been able to publish their findings only now because litigation has been completed. In 1994, 148 of 180 people who claimed compensation won amounts ranging from pounds 600 to pounds 11,000, with an average payment of pounds 2,000. Charles Pugh, a barrister who represented the victims until other commitments forced him to step aside shortly before they settled, said the BMJ study cast new light on the incident.
Other victims now pursuing the case would run into problems. Under the 1980 Limitation Act, litigation should be initiated within three years. Mr Pugh said: "The court does have a discretion to extend the limitation period where it is fair and just. Bearing in mind that South West Water was convicted of public nuisance, it would not appear that establishing liability was difficult. The whole issue would be in establishing causation, something the new evidence goes to the heart of."