ONS reported a One Number Census Count for the UK Population of 58,789,194. This proved highly controversial since it suggested a smaller population than had been expected from the official mid-year estimates from the Registrar General. Moreover, the shortfall varied considerably between areas, being greatest in certain urban districts including Manchester, Bristol and Westminster.
As in 1991, age sex ratios indicated that the shortfall was concentrated among young males. In 1991, Census data were adjusted to 'correct' this apparent mistake. This time however, based on greater confidence in the Census coverage Survey, the official conclusion was that the 2001 Census was right and the Register Generals official estimates were wrong, and that the 'missing' individuals were not there to be counted.
With the distribution of many government resources based on the size of population counts, the decision to accept the accuracy of the ONC has proved contentious, especially for those local authorities like Manchester and Westminster where the ONC counts show the greatest shortfalls from previously accepted estimates.
The explanation offered for the discrepancy was essentially that the accuracy of the RGs mid-year population estimates had been adversely effected by inaccurate estimates for migration. In particular, data for international migration (based on the International Passenger Survey) had failed to capture the extent of emigration among the young male population.