Migrationwatch said there was little incentive for many unemployed benefit claimants to get a job because they would hardly be better off for working.

After analysing the benefits system, the group said a family with two children, for example, would get only £30 a week more working on the minimum wage rather than claiming benefits.

It claimed this was one of the reasons more than a million immigrants had come to work in Britain in the last 10 years, despite a pool of millions more Jobseekers Allowance claimants already in the UK.

Migrationwatch chairman Sir Andrew Green said: "We keep hearing that we need immigrants to do the jobs that the British won't do.

"It has been suspected for some time that benefit levels are a real disincentive to take work that is on offer and our research spells out why this may be so."

Sir Andrew said other factors to take into account included the flexibility of many immigrants who are looking for work.

But he added: "An important factor is that wages are now so close to benefits that there is very little financial incentive for unskilled British workers to find a job. By contrast, Poles have very strong financial motivation.

"On the minimum wage in Britain they are earning four to five times what they would earn at home and, by living in multi-occupancy, they can afford to send considerable sums of money back to their families - according to the National Bank of Poland, Polish migrants in the UK are sending home about £9 million a day."

Work and Pensions Secretary Peter Hain said unemployment benefit claimants had fallen since Labour came to power and insisted that immigration had assisted growth.