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Below are specific findings about unauthorized immigrants in the United Kingdom. The findings come from a Pew Research Center report about unauthorized immigrants in Europe. 
An estimated 800,000 to 1.2 million unauthorized immigrants lived in the United Kingdom in 2017, according to new Pew Research Center estimates based on the latest available data. This number is little changed since 2014.
The UK’s unauthorized immigrant estimate accounts for roughly a quarter of the estimated 3.9 million to 4.8 million unauthorized immigrants living in the 32 European Union and European Free Trade Association (EFTA) countries at the end of 2017. The total number of unauthorized immigrants living in EU and EFTA countries increased since 2014, when it stood at 3.0 million to 3.7 million.
Unauthorized immigrants are non-EU-EFTA citizens who entered the UK without permission and have an unlikely path to permanent residency. The population also includes those who may have entered lawfully but overstayed a visa. Asylum seekers waiting for a final decision in their case are included in the unauthorized immigrant population estimates. Their future status is uncertain as rejection rates are high.
Pew Research Center estimates for the United Kingdom’s unauthorized immigrant population used the “residual” method, an indirect way to estimate the size of this population. It is the same method used by the Center in the United States to estimate the size of its unauthorized immigrant population.
The number of unauthorized immigrants in the UK is in line with previous estimates and recent migration trends. In 2007, a Greater London Authority and London School of Economics study placed the number of unauthorized immigrants between 417,000 and 863,000. Ten years later, after hundreds of thousands additional migrants from non-EU-EFTA countries have entered and stayed in the UK, the Center’s 2017 estimate of 800,000 to 1.2 million unauthorized immigrants is consistent with these recent migration trends.
For more information about the unauthorized immigrant population in the United Kingdom or other EU-EFTA countries, see the Center’s full report.
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About Pew Research Center Pew Research Center is a nonpartisan fact tank that informs the public about the issues, attitudes and trends shaping the world. It conducts public opinion polling, demographic research, media content analysis and other empirical social science research. Pew Research Center does not take policy positions. It is a subsidiary of The Pew Charitable Trusts.