The number of undocumented immigrants in the U.S. has decreased in more than a decade and for the first time dropped below 11 million.

According to a new study by the Center for Migration Studies, undocumented immigrants totalled 10.9 million in 2014, the lowest since 2003, dipping below 11 million since 2004, according to the Los Angeles Times.

The decrease is in contrast to the image being portrayed in the Republican campaign trail that illegal immigration is on the rise, the report said.

The study showed that the undocumented population has fallen by more than one million since 2008 when it reached 12 million.

"Clearly, however, the sharp reduction in arrivals from Mexico in the past 10 years has played a significant role," the study read.

Republican presidential bet Donald Trump once claimed that illegal immigration rates are "beyond relief" and said these people are "just pouring across the border."

He vowed to enforce mass deportations and build a giant border wall.

The study showed that the number of undocumented Mexicans in the U.S. has decreased by about 612,000, or 9 percent.

Mexican immigrants in California declined by about 250,000 from 2010 to 2014 and the total population of undocumented immigrants fell by 318,000 during the period.

The study also showed that the number of immigrants entering the U.S. illegally from Central America has increased.

According to Manuel Pastor, a sociology professor at the University of Southern California, the decrease in the number of the undocumented could be due to the decline in the U.S. economy and other factors.

He described Trump's threat on illegal immigrants as "detached from reality" and suggested that it could be a product of the Republican presidential primary system in which the candidates must appeal to the party's most conservative and mostly white voters.

"Undocumented immigration has been a significant political issue in recent years, and is likely to remain so throughout and beyond the presidential election year of 2016. One reason for the high and sustained level of interest in undocumented immigration is the widespread belief that the trend in the undocumented population is ever upward," wrote CMS' Robert Warren, adding that the belief is "mistaken."