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HUNDREDS of thousands of short-term immigrants are not included in official statistics because the system for counting population changes is "not fit for purpose,'' a parliamentary inquiry has found.

The counting system is so unreliable that it is not even possible to know the true population of Britain, MPs will report today

The Treasury Sub-Committee warns that the failure to correctly count the number of foreigners coming into Britain is undermining Government policies and putting unfair pressure on many local authorities and taxpayers.

After taking evidence from officials, council leaders and academics, the MPs identified a fundamental weakness with the International Passenger Survey, the main tool used by the Office of National Statistics (ONS) to measure immigration and emigration.

The survey, which is based on around 4,000 interviews each year, was designed to monitor tourism and business travel, but it has come to play a central role in immigration policy.

"It is clear from the evidence we have received that the survey is not fit for this new purpose,'' the MPs say.

The ONS is overhauling its collection methods to track the movements of short-term migrants. But the MPs found that there are still major shortcomings, suggesting that hundreds of thousands of people are being omitted.

The ONS estimated that there were only 43,000 short-term migrants in England and Wales in the year to June 2005, with just 16,000 of these in Greater London. But the MPs point out that other measures of the workforce suggest a much higher number: there were 235,640 new national insurance number registrations in Greater London alone during 2005/06.