The study, of cancers among the children of workers employed at Sellafield, has confirmed the existence of a cluster of cases in the village of Seascale, just three kilometres (1.8 miles) from the plant.
After spending three years studying the evidence, the Health and Safety Executive found 'a strong statistical association' between each case and the radiation dose that the father had received before the child's conception.
But despite being the most intensive examination of the issue so far, the HSE study has deepened the mystery surrounding the Sellafield leukaemia cases. The effect is confined to Seascale and is not found among equally highly exposed fathers resident in west Cumbria. Moreover, only the children of fathers who started work at Sellafield before 1965 are affected.
Eddie Varney, who directed the study for the HSE, said that it offered support to the theory that as populations of workers moved into the area from outside, they might have brought some cancer-causing infection or virus with them. 'We cannot find any single cause that satisfactorily explains what we see. It is difficult to deny a role for population mixing, but we find it hard to rule out radiation in the case of Seascale. But radiation alone cannot explain Seascale.'
The HSE investigation was prompted by a controversial study published in 1990 by the late Professor Martin Gardner. This first linked childhood leukaemia and non-Hodgkins lymphoma with the radiation dose which the child's father had received before the child was conceived. Mr Varney said: 'My impression is that it (the HSE study) does not support the Gardner report.'
The report came only 10 days after a High Court judge said radiation exposure of workers at Sellafield was not responsible for their children's cancers.
Yesterday Martyn Day, the solicitor for the families, said: 'The HSE report gives us very strong grounds for an appeal. But I am very disappointed that its findings were not made available earlier. This goes a long way to give us the support we needed during the case. It answers a lot of issues that clearly troubled the judge.'
Mr Justice French had ruled that the Gardner report - the scientific study of the cancer clusters at nearby Seascale, upon which the families depended - was flawed and unsupported by more detailed scientific studies. 'This new report shows that Gardner was right,' Mr Day said.