British government officials have backed the methods used by scientists who concluded that more than 600,000 Iraqis have been killed since the invasion, the BBC reported yesterday.

The Government publicly rejected the findings, published in The Lancet in October. But the BBC said documents obtained under freedom of information legislation showed advisers concluded that the much-criticised study had used sound methods.

The study, conducted by researchers from Johns Hopkins University in Baltimore and the Al Mustansiriya University in Baghdad, estimated that 655,000 more Iraqis had died since March 2003 than one would expect without the war. The study estimated that 601,027 of those deaths were from violence.

The researchers, reflecting the inherent uncertainties in such extrapolations, said they were 95 per cent certain that the real number of deaths lay somewhere between 392,979 and 942,636.

The conclusion, based on interviews and not a body count, was disputed by some experts, and rejected by the US and British governments. But the chief scientific adviser to the Ministry of Defence, Roy Anderson, described the methods used in the study as "robust" and "close to best practice". Another official said it was "a tried and tested way of measuring mortality in conflict zones".