WASHINGTON - Between 3.9 million and 4.8 million unauthorised immigrants lived in Europe in 2017, mainly recently arrived young men, numbers slightly lower from the previous year, the US-based Pew Research Center said Wednesday.

The figures "accounted for less than one percent of Europe's total population of more than 500 million," Pew said.

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For the study, an unauthorized migrant was defined as a foreigners who entered the country illegally, those whose residence permit has expired, are under expulsion orders, or asylum seekers with pending decisions, Pew said.

Pew analyzed data from the European Union and European Free Trade Association countries (Switzerland, Liechtenstein, Norway and Iceland).

The US researchers said the number of unauthorized migrants in Europe peaked in 2016 at between 4.1 and 5.3 million.

In 2017, 30 percent of the migrants were from the Asia-Pacific region, including Afghanistan).

The next largest group -- 23 percent of the total -- were from European countries not in the EU or the European Free Trade Association.

Another 21 percent were from North Africa and the Middle East; 17 percent from sub-Saharan Africa, and eight percent from the Americas.

While each European nation had some unauthorized immigrants in 2017, "the largest numbers were in Germany and the United Kingdom, amounting to about half of Europe's total."

Substantial numbers also lived in Italy and France, although France had a relatively lower number possibly because some unauthorized migrants can obtain residency after living there for several years, Pew said.

Italy is the first stop for many migrants crossing the Mediterranean -- and the country's relatively large population of unauthorized migrants (700,000) may be due to tight frontier controls at bordering nations, according to Pew.

Together the four countries are "home to more than two-thirds (70 percent) of Europe's unauthorized immigrants," the report said.

Most of these unauthorized migrants -- 56 percent -- had lived in European countries for less than five years as of 2017, and about half were awaiting a decision on their asylum applications.

Most of the migrants were males under the age of 35 who had arrived in the previous five years or less.

Half of the migrants were seeking asylum, compared to 20 percent in the United States, according to Pew.

In the US case, the undocumented migrant population is between 10.3 and 10.7 million and represents about three percent of the population.

The overwhelming majority come from Latin America, and about half come from Mexico.