Editorial note to readers (March 19, 2025):
This project estimated the number of unauthorized immigrants living in 32 European countries during the mid- to late-2010s. We used what were then the best available data sources from different governments and from the European Union. After our report was published in November 2019, researchers at the University of Oxford’s Migration Observatory identified a problem with the official data from the United Kingdom’s Office for National Statistics (ONS) on the number of legal immigrants residing in the UK.
The ONS data on legal immigrants that we used in our report did not include those with “indefinite leave to remain” (ILR), a group legally residing in the country. Including the ILR group among legal UK residents results in lower estimated total numbers of unauthorized immigrants for both the United Kingdom and Europe in 2017 and other years. The updated estimates for the number of unauthorized immigrants in 2017 (without waiting asylum seekers) are 700,000-900,000 for the UK and 2.8 million-3.5 million for Europe. Updated estimates can be found in this table but are not reflected elsewhere in this fact sheet.
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.
Additional findings for the United Kingdom
- The number of unauthorized immigrants in the UK was 800,000 to 1.2 million in 2017, similar to 2014. With such a wide range, however, it is possible that the unauthorized immigrant population did change during this period. The UK was not a top destination for the recent 2015 surge of asylum seekers in Europe. As a result, the bulk of the UK’s unauthorized immigrants were likely people who had overstayed their visas or asylum seekers who remained in the UK after not obtaining refugee status.
- Among those with non-EU-EFTA citizenship living in the UK in 2017, more were authorized (1.2 million to 1.6 million) than unauthorized (800,00 to 1.2 million) immigrants.
- About half (52%) of the UK’s unauthorized immigrants living in the UK in 2017 were from Asian countries. One-in-five (20%) were from sub-Saharan African countries, while about one-in-ten (11%) were from Middle East-North Africa countries. The remaining share (16%) had nationalities from the Americas and other European countries besides EU-EFTA countries.
- More than half (57%) of unauthorized immigrants living in the UK in 2017 are estimated to have lived in the country for five years or longer. This is different from unauthorized immigrants overall in Europe, where the majority have lived in EU-EFTA countries less than five years.
- Almost equal shares of unauthorized immigrants in the UK were male (48%) and female (52%) in 2017. The majority (58%) in 2017 were under 35 years old.
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.
Updated estimates (March 19, 2025):
