The number of U.S.-born workers has steadily decreased since 2000, causing the labor force participation rate to reach its lowest rate since records have been maintained, according to a new report.

The Center for Immigration Studies released a study to mark the one-year anniversary of the passage of the Senate Gang of Eight immigration bill on Friday, discrediting the notion that adding millions more workers through comprehensive immigration reform is necessary due to a coming labor shortage.

Analyzing statistics from the Census Bureau, the report found that native-born U.S. citizens have struggled to compete with both illegal and legal immigrants for more than a decade.

"The findings show that employment growth has been weak over the last 14 years and has not kept pace with population growth and new immigration," the report said. "Among the working-age (16 to 65), what employment growth there has been has entirely gone to immigrants (legal and illegal)."

"This is truly remarkable because natives accounted for two-thirds of overall population growth among the working-age population," it said.

According to the Census Bureau’s Current Population Survey, there are 17 million fewer working-age natives with a job than there were in 2000. That year there were 131.9 million people in the working age population who were employed. The number has only grown by 5.6 million since, despite the addition of 25.7 million workers due to population growth.

"In short, natives accounted for two-thirds of the growth in the number of potential workers, but none of the growth in the number of actual workers," the report said.

The labor force participation rate for working-age natives also shows a "steady deterioration," declining from 77.1 percent in 2000 to 71.5 percent today.

"Among working-age natives, labor force participation is the lowest it has been since the CPS began identifying immigrants and natives in 1994," the report said.

Immigrants have fared much better, as a "disproportionate share of employment growth went to immigrants" since 2000.

"All of the net increase in employment went to immigrants in the last 14 years partly because, even before the Great Recession, immigrants were gaining a disproportionate share of jobs relative to their share of population growth," the report said.

According to the Census Bureau data, the number of employed immigrants has risen by 5.7 million since 2000.

Prior to the recession between 2000 and 2007, the number of natives holding a job increased 2.9 percent while the number of immigrants with jobs increased 28.7 percent.

Furthermore, since the jobs recovery began in 2010, 43 percent of employment growth has gone to immigrants, the report said.

"The findings in this report are shocking, and represent a dramatic indictment of immigration policy in Washington D.C.," Sen. Jeff Sessions (R., Ala.) said in a statement. "This report also underscores the economic catastrophe that would have ensued had the Gang of Eight’s legislation, passed in the Senate one year ago today, been moved through the House and signed into law."

Young American-born citizens have especially felt the impact of unemployment.

"Among young natives 16 to 24 years old the share holding a job was 12.4 percentage points lower in 2014 than it was in 2000," the report said.

8.7 million native college graduates, 17 million with some college, and 25.3 million with only a high school diploma are not working.

"[Y]oung people, particularly the less educated, have not found jobs over the last 14 years," the report said.

The Center for Immigration Studies points to a variety of factors for why immigrants have done better in the U.S. job market than native-born Americans, including government programs that incentivize the hiring of legal and illegal immigrants.

The report references the Summer Work Travel Program, which allows employers to hire temporary workers without having to pay into Social Security and Medicare. The H1-B visa program also does not allow immigrants to change jobs easily, allowing employers to take advantage of the program.

"Immigrants may also be more willing to work off the books, for lower pay, or endure worse working conditions than natives, causing employers to prefer them as workers," the report said.

While noting that the Gang of Eight bill would make the employment situation worse for Americans, the report adds that the current system is impeding job growth.

"[G]iven that the labor force participation for natives shows an almost uninterrupted 14-year decline, it seems unlikely that labor force participation will ever return to the 2007 or 2000 level, particularly if immigration stays at its current level," the report said.

"There is no doubt that a long, sustained period of high immigration, combined with increased automation and the offshoring of jobs, has produced a loose, low-wage labor market," Sessions said. "In spite of this, the president continues to champion legislation that would place further substantial downward pressure on wages."

"The sensible, conservative, fair thing to do after 40 years of record immigration is to slow down a bit, allow assimilation to occur, allow wages to rise, and to help workers of all backgrounds rise together into the middle class," he said.