Irregular, unauthorized, or undocumented migration is the practice of crossing an international border without official permission from the authorities. Irregular migration is not synonymous with illegal immigration because irregular travel in order to seek asylum is not a crime. According to the European Commission, "the term 'illegal migration' should be avoided, as some irregular migrants are not attempting illegal immigration. Being in a country without the required papers is, in some countries, not a criminal offence but an administrative infringement." In 1993, the UK House of Lords ruled that an asylum seeker who had forged papers in order to enter the UK had not entered the country illegally.
List RankMetropolitan areaCountryEstimated irregular migrant populationYearNotes1New York metropolitan area1,100,0002016Pew Research Center.2Metropolitan Lima1,000,0002025Reuters; National Superintendency of Migration.3Los Angeles metropolitan area925,0002016Pew Research Center.4Madrid metropolitan area700,0002026Reuters.5Greater London390,000–585,0002017Pew Research Center.6Johannesburg metropolitan area500,0002026Associated Press.7Santiago metropolitan area300,0002023United Nations High Commissioner for Refugees.8Chicago metropolitan area180,0002019Migration Policy Institute.9Houston metropolitan area175,0002019Migration Policy Institute.10Dallas–Fort Worth metroplex170,0002019Migration Policy Institute.11Miami metropolitan area150,0002019Migration Policy Institute.12Paris metropolitan area150,0002017Pew Research Center.13Milan metropolitan area120,0002017Pew Research Center.14Greater Toronto Area100,0002024Government of Canada.15Athens metropolitan area100,0002017Pew Research Center.
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