One million immigrants from Eastern Europe have come to Britain since 2004 - but half of them have already gone back home, a report showed on Wednesday.
A think-tank study also suggested that future numbers of immigrants from the 10 new European Union members would slow as economic conditions improved in their home countries.
The Institute for Public Policy Research (IPPR) findings highlighted the gulf between the actual cross-border flow and initial government predictions that only a few thousand would be tempted to seek work in the country.
"Four in ten of the returned Polish migrants we surveyed think that better employment prospects in Poland would encourage Poles living in the UK to return to Poland for good," it said.
Eight states joined the European Union in May 2004 - Poland, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Slovakia and Slovenia. Bulgaria and Romania joined in 2007.
The research, released to the BBC, showed there were 665,000 people from the 10 new EU members living in Britain in the last quarter of 2007.
"We estimate that some 30,000 fewer migrants arrived in the second half of 2007 as did in the second half of 2006," the IPPR said.
With other European Union countries changing their restrictions on new members, the think-tank forecast that many more would-be migrants might be tempted to move elsewhere.
Declining birth rates might mean there was a smaller pool of migrant workers and the weakening of the sterling has resulted in the gap narrowing in potential earnings.
The pound, it said, had fallen by about 25 percent against the Polish Zloty since early 2004.
The government had originally predicted that up to 13,000 migrant workers would arrive after the 2004 accession but admitted later that its calculations had been well wide of the mark.
They have fanned out across the economy, working as everything from plumbers and waiters to seasonal fruit pickers.
The new wave of immigrant workers had sparked fears that Britain's health and education services could be overwhelmed.
But a 2005 report from the think-tank concluded after the first wave of immigration: "Our research shows that immigrants make an important fiscal contribution to the UK and pay more than their share. They are not a drain on the UK's resources."
