Around 1.3 million citizens of eight eastern European countries are living in the UK, new analysis shows.
This is roughly 100 times the number of British ex-pats living in the group of states, which joined the EU in 2004.
The figures were disclosed in an official report on migration between the UK and the so-called "EU8" countries - Poland, Lithuania, Czech Republic, Hungary, Slovakia, Slovenia, Estonia and Latvia.
Poland accounts for the highest number of EU8 nationals in the UK, with an estimated 813,700 citizens - the largest population for any overseas country.
This echoes previous findings pointing to the growth of the Polish community in Britain. Last year, data showed Poland had overtaken India for the first time as the most common overseas country of birth.
Statistics also suggest there has been a rise in the number of overseas nationals claiming the UK state pension from countries in the region.
The new study from the Office for National Statistics (ONS) cited figures from the Department for Work and Pensions showing that there were around 6,000 people in EU8 countries receiving a pension at the end of last year.
The number of pension recipients in Poland has risen most sharply, with around 2,900 in November, which is twice the number recorded six years earlier.
"As there is not a corresponding rise in the number of British citizens aged over 65 years living in EU8 countries, it is likely that the increase is due to EU8 migrants returning home and claiming the state pension they are eligible for by working in the UK," the paper said.
Statisticians reported that the overwhelming majority of EU8 nationals in the UK are of working age.
On average, 80% of those aged 16 to 64 were employed at the point they were surveyed in 2013 to 2015. EU8 citizens made up 3% of the overall national workforce.
Just under a third (32%) of EU8 workers were in "elementary" occupations, with distribution, hotels and restaurants, and manufacturing the most common industries. Two in five EU8 nationals were overqualified for the job they were doing.
The paper said there are an estimated 14,100 UK citizens living in EU8 countries, with 42% residing in the Czech Republic.
Discussions over the status of EU nationals in the UK, and British expats in Europe, have been at the centre of early Brexit negotiations.
Emma Rourke, ONS Director for Public Policy and Analysis, said: "This report adds to the complex picture of migration across the EU.
"It shows how the population profile among migrant populations compares, and often differs markedly in age make-up, with the overall population of the host countries.
"This has clear implications for future policy-making across a range of areas."