Sriskandarajah said that although migration from the new EU states had happened on a "staggering scale" it seemed to have been largely positive for all concerned.
Eight new states joined the European Union in May 2004; Poland, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Slovakia and Slovenia.
Bulgaria and Romania entered the bloc in 2007.
|"There is a risk that critical sectors in the UK economy ...|
will have great difficulty in finding workers to fill vacancies if the tide turns quickly and significantly"
Danny Sriskandarajah, report author
The research 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 report said.
The weakening of the British pound has resulted in the gap in potential earnings narrowing between the UK and the new member countries.
The British pound has fallen by about 25 per cent against the Polish zloty since early 2004.
"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," the report said.
The findings highlighted the gulf between the large number of people actually entering the UK and initial British government predictions that only a few thousand would be tempted to seek work in the country.
The British government had originally predicted up to 13,000 migrant workers would arrive after the 2004 accessions, but admitted later its calculations were wide of the mark.
The report said that the immigrants had fanned out across Britain, working as everything from plumbers and waiters to seasonal fruit pickers.
The number of workers arriving from Eastern Europe had sparked fears Britain's health and education services could be overwhelmed.
However, a 2005 IPPR report concluded "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".
Asked if a sudden exodus could cause problems for industries like fruit-picking, Sriskandarajah said: "There is a risk that critical sectors in the UK economy, especially in rural areas, will have great difficulty in finding workers to fill vacancies if the tide turns quickly and significantly."
He suggested the government might have to ease immigration restrictions for countries outside the European Union.