The government's migration figures are "not fit for purpose" and an urgent improvement is needed if resources are to be allocated efficiently, a Treasury committee report said on Friday.
The report said the government had no way of knowing accurately the size of the population from one year to the next, making it difficult for local authorities to budget.
"It is now impossible to estimate accurately the UK population today," said Michael Fallon, chairman of the Treasury sub-committee overseeing official statistics.
"Unreliable statistics make planning impossible. We call on the government to improve the population count as a matter of urgency."
Government statisticians have themselves acknowledged the limitations of current methods of data collection and are studying how best to focus their resources.
At present, population estimates are drawn from a variety of imperfect sources. There has been widespread concern that they do not accurately reflect the wave of migrants to Britain since European Union enlargement eastwards in 2004.
The committee expressed particular concern about the statistics office's reliance on the International Passenger Survey - based on interviews with a random sample of travellers passing through ports and airports - which was originally designed for tourism and business travel purposes.
It urged the Statistics Authority to devise a better way of estimating the population as an "immediate priority".
