A cross-party committee of MPs has expressed concern that low census response rates in some parts of London may have undermined the entire process.

The public accounts committee said that rates of return were lower than 80 per cent in 10 London boroughs.

This led to some local authorities questioning the results for their areas, the MPs note.

The committee also found that more than £10 million was wasted on staff employed to chase forms. It says the Office of National Statistics failed to estimate the high number of forms which were returned by post.

This meant it spent £11.3 million on unnecessary staff recruited to collect forms by hand.

The report says the ONS "was not adequately prepared to take advantage of the higher than expected rate of return by post".

Throughout the country as a whole some 88 per cent were sent by post - compared with the 70 per cent expected.

"As a result, the decision to keep the staff in the field during May contributed to some £11.3 million being spent which could have been avoided," said the study.

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

"It is worrying therefore 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 £11 million."