Kenneth Rokison QC, for British Nuclear Fuels, said the report's limitations represented a 'fundamental flaw' in the case being put forward by two families who say contamination from the plant caused leukaemia.
He rejected a suggestion made by the families' counsel on Monday that the company was cavalier in its approach to accuracy. 'That assertion is not only resented by BNFL but is emphatically denied,' he told Mr Justice French.
His comments came on the second day of a hearing that is expected to last several months into damages claims by Viven Hope and Elizabeth Reay, both of Cumbria. Ms Hope, whose father, David, worked at Sellafield, is claiming pounds 125,000 after developing non-Hodgkin's lymphoma, from which she has made a partial recovery.
Mrs Reay, whose husband, George, also worked at the plant, is suing BNFL for pounds 150,000 for the death of her 10-month-old daughter Dorothy in 1962. Both families say the cancers resulted from genetic damage caused by their fathers' exposure to radiation.
Central to their case is a 1990 study by Professor Martin Gardner, of the University of Southampton, who found 'statistically significant' numbers of childhood leukaemias among the children of Sellafield workers.
But yesterday, Mr Rokison said the Gardner report, which depended on very small numbers, had included a case which ought to have been excluded. 'Its fragility is demonstrated by the fact that . . . if one case were to be removed or re-classified for any reason, statistical significance would probably disappear,' he said.
The Gardner report was not supported by any other epidemiological study and was biologically implausible. 'It has not been shown that leukaemia has a genetic or at least a strong genetic component,' Mr Rokison said. A study of 76,000 children born to the survivors of the atomic bombs at Hiroshima and Nagasaki had found no significant increase in the risk of leukaemia.
He also challenged allegations that BNFL had badly underestimated - and then covered-up - the levels of radiation received by workers at the plant and people living near it.
Benet Hytner QC, for the plaintiffs, had said the company failed to publicise research by one of its own experts, Professor Steve Jones, showing that plutonium discharges into the atmosphere before 1984 were almost 20 times higher than previously thought.
Yesterday, Mr Rokison said Professor Jones' findings had been cautious and extremely pessimistic, a fact confirmed by a second, independent study.
He also rebutted criticisms that BNFL had misled Professor Gardner over radiation levels to which employees had been exposed.
The hearing continues.