2,800 staff were overpaid

The Commons Public Accounts Committee (PAC) questioned the "accuracy" of results from the 2001 survey after it claimed the population of England and Wales was 900,000 lower than previously thought.

Less than 80% of people in 10 Inner London boroughs returned their census forms, with only two thirds responding in Kensington and Chelsea.

Wasted time in pursuit of forms that had already been returned cost the taxpayer over �11m Edward Leigh

The census, run by the Office for National Statistics, overspent by more than �11m because it had not adequately prepared for the higher than expected return of forms by post, the MPs said.

Staff were overpaid by half a million pounds and ONS "failed to follow its own procedures" by taking on consultants without competition at a cost of �485,000.

'Worrying'

Payments to 23,000 field staff were "significantly delayed" and 2,800 were overpaid by a total of �500,000.

Edward Leigh, Tory chairman of the committee, said: "Accurate data from the 2001 census are essential if taxpayers' money is to be targeted where it should be.

"It is worrying that the response rates achieved for some areas, including Inner London boroughs, have been so low that the accuracy of the results is being questioned.

"Changes in approach for the 2001 census, particularly the return of completed returns by post, have allowed more data to be recorded than before whilst employing fewer field staff.

"But wasted time in pursuit of forms that had already been returned cost the taxpayer over �11m."

The MPs said ONS should examine why it failed to bring response rates "anywhere near" national levels in the hard-to-count city areas.

While outsourcing helped bring important innovations to deliver the census, the committee concluded costs had increased and results were damaged by failures in its implementation.