Migration policy figures missing, say Oxford academics
British immigration policy suffers from "fundamental uncertainty" because nobody knows for sure how many people are coming and going, say academics.
Oxford University's Migration Observatory said there were "serious problems" with official figures.
Academics at the university said they had identified 10 key problems with measuring migration in the UK.
The report says some figures contradict each other and little is known about immigrants' impact on public services.
The university launched the Migration Observatory saying there needs to be more independent and "authoritative" evidence available on how migration affects the UK.
But in one of the unit's first reports, the authors said there were substantial discrepancies between the major sources used to calculate movements of people and why they come and go.
Those discrepancies meant that in 2009 there was a difference of 400,000 people between two of the three methods that can be used to count immigrants from outside Europe.
On emigration, the report said that the only source of information was a survey of some 2,000 departing passengers. One of the problems with this survey, said the authors, was that it did not record useful information, such as whether some of those counted as emigrating British nationals were originally foreign-born.
In turn, the problems with immigration and emigration statistics had a knock-on effect on attempts to accurately calculate the net effect of migration on the population.
Dr Martin Ruhs, director of the Migration Observatory, said: "There are many disagreements about migration in the UK, but one thing that unites everyone is that for many years there have been serious problems with the evidence base.
"Reform of immigration policy is currently a major government priority, which clearly requires solid evidence on which to base changes to the system."
Migration experts have long lobbied the Home Office to improve the statistics - and Immigration Minister Damian Green recently described some figures provided to Parliament as "rough and ready". The department is pressing ahead with the massive e-borders programme which aims to count accurately the number of people coming to and going from the UK.
The coalition government is introducing its first immigration cap on 6 April, pledging to bring net migration down from almost 200,000 to "tens of thousands" by the end of the Parliament.
But in a a separate report, Migrationwatch UK, which campaigns for tougher controls, predicted that the immigration cap would only have a "limited effect".
It predicted the policy would be undermined by allowing companies to move existing staff in to the UK and a further decision to allow foreign students to stay on after graduation if they have a skilled job offer.
"The continuing flow of economic migrants makes it all the more important that the government should break the present almost automatic link between economic migration and settlement," said the pressure group.
"There must also be much more effective measures to ensure the return of workers when their visas expire."