Eastern European nationals have settled more widely throughout the UK than has happened during previous waves of migration, research suggests.
The Institute for Public Policy Research examined their impact on the UK after EU expansion in 2004 and 2007.
It said even areas which had not traditionally attracted migrants, such as Scotland and south-west England, had noticed a "significant" influx.
Poles are now the largest foreign national group in the UK.
They had been the 13th largest group in early 2004, the report said, but had replaced people born in India, many of whom now have UK citizenship, as the biggest group.
Regular visits home
The research, released to the BBC, looked at migrants who came from eight countries that joined the European Union in May 2004 - Poland, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Slovakia and Slovenia.
It also included migrants from Romania and Bulgaria, which joined the EU in 2007.
The research by the IPPR, a Labour-leaning British think tank, estimated that about one million migrant workers had come to the UK from 2004 accession countries, but that about half this group had already left the UK.
The IPPR also said post-enlargement migration was "very different" from previous migration to Britain.
"In contrast to previous migrants, it is financially and logistically possible for migrants from the new EU member states to come to the UK on a temporary or seasonal basis, and to regularly visit home while living in Britain," it said.
The IPPR looked at the Labour Force Survey, national insurance number applications, the Workers Registration Scheme - applicants are required to register on the scheme as soon as they start working in the UK - and the International Passenger Survey.
A Polish guide gives a tour of a Scottish distillery
It also surveyed Poles who had returned to their homeland after working in the UK, as well as applying its own projections.
While the highest number of national insurance applications by migrants since 2004 was in London and south-east England, all regions had received significant numbers of post-enlargement migrants, the IPPR said.
"In 2007 Polish [national insurance number] recipients were registered in every local authority in Britain."
In the Highlands, Perth and Kinross, Edinburgh and Glasgow between 5,000 and 20,000 workers had registered in each area since 2004 - although not every worker has to register, so it is not an accurate figure of current worker numbers.
The IPPR estimated there were between 10 and 29 Eastern European workers per 1,000 local residents in those Scottish areas.
IPPR's conclusions may provide solace to those who fear immigration is too high but will be greeted with alarm by some businesses
Parts of Norfolk and Lincolnshire have attracted many migrant workers, with Boston having an estimated 90 workers per 1,000 local population, according to IPPR estimates, second only to the City of London at 307 EU workers per 1,000.
But when it came to reasons for coming or going, the report said "the vast majority of Polish migrants come to the UK for economic reasons, but leave because they miss home or want to be with their friends and family in Poland".
Other findings included:
Three-quarters of all nationals from the 10 countries resident in the UK in 2007 were aged 16 to 39
At 84%, the employment rate among post-enlargement migrants is among the highest of all immigrant groups, and is nine percentage points higher than the UK-born average
Very few post-enlargement migrants claim state benefits (only 2.4% of those registering for National Insurance numbers between May 2004 and December 2007 did so in order to claim benefits)
East European migrants work on average four hours longer per week than UK-born workers (46 hours compared with 42 hours)
Accession has also had an impact at the pub - before 2004, Polish beers were not widely available in the UK.
Now some 44 million pints of Lech and Tyskie, Poland's two leading brands, are sold annually in the UK.
UK sales of Polish beer have increased to 44 million pints a year
According to the research, there were 665,000 nationals from all 10 countries living in the UK in the last quarter of 2007.
This was an increase of 548,000 since the first quarter of 2004, just prior to the first eight countries joining the EU.
The government had underestimated the number of migrants post-expansion, saying that between 5,000 and 13,000 would arrive after 2004.
By 2006, Home Office minister Tony McNulty admitted that the government was "in the dark" over arrivals - in fact 293,000 immigrants had applied for work permits in the first 18 months.
The IPPR, established in 1988, says it aims to promote social justice, democratic participation and sustainability in government policy through its research and analysis.
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