The number of illegal aliens that have arrived into the United States from Mexico has declined so sharply during the past decade that Mexicans are no longer a majority of unauthorized immigrants living in the country, according to a report by the Pew Research Center.

"Mexicans remain a much larger percentage of all unauthorized immigrants than those from any other birth country," the report stated on Wednesday. "But their 47 percent share of US unauthorized immigrants in 2017 amounted to less than a majority for the first time since the beginning of a long era of growth in illegal immigration a half-century ago."

The number of Mexican unauthorized immigrants declined because more left the United States that arrived, the report said. The number of Mexican unauthorized immigrants has fallen by 2 million since its peak of 6.9 million in 2007 and was lower in 2017 than in any year since 2001. 

PEW demographer Jeff Passel explained that the estimates are based on data from the US Census Bureau, adjusted with information from Census Bureau studies of estimated undercounts plus data from Mexico.

The report also provided an estimate of the total illegal alien population in the US, 10.5 million in 2017, which is less than the 11 million or 12 million often cited in political discourse.

However, US officials expect the population of illegals to swell this year, with more than 1 million migrants mainly from Central America expected to cross the US southwest border.