International migration is eroding Britain's skills base with an exodus of professionals matching the arrival of low-skilled foreign immigrants
The number of Britons emigrating has jumped in recent years, with a growing proportion leaving professional or managerial jobs to work overseas. By contrast, the number of immigrant workers - many of them manual workers - has risen sharply.
The extent of the problem will be revealed in the annual report on international migration from the Organisation for Economic Co-operation and Development (OECD), to be published this summer.
The section on Britain has been written by John Salt of University College London, an expert on migration and an adviser to the OECD and the European Union.
In it, he says: "The evidence suggests that migration flows are tending towards a deskilling of the UK labour market, which is gaining manual and clerical workers but losing professionals and managers."
The finding will call into question claims by ministers that immigration boosts the economy by helping employers to tackle skills shortages.
MPs and trade unions have already claimed that the arrival of migrant workers is driving down pay and reducing job opportunities for the established work force.
Prof Salt's report is also critical of the new points-based system for assessing the skills of would-be migrant workers, due to be launched by the Home Office later this year.
Under the new system, checks on candidates will be carried out by entry clearance officers and case workers based in the countries where the applications are made.
However, the report says: "A major concern is about the capacity of such a geographically distributed system to meet the criteria of objectivity, consistency and transparency."
Until recently, business leaders were broadly supportive of the Government's position on migration.
However, a report last month by the British Chambers of Commerce revealed that seven out of 10 of its members are now opposed to unchecked immigration.
David Frost, the organisation's director general, said: "Outside London, we are increasingly seeing large numbers of white, unemployed males wandering the streets.
"This is not pointing to a bright and positive future. We need to engage with these people once more and get them trained up.
"Immigration is not solving today's problems but actually postpones them."
Between 2000 and 2005, a net total of 272,000 Britons emigrated, while a net total of 639,000 non-Britons moved to the UK.
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