That growth, coupled with births among current residents, would boost the nation's population to 468 million from its current level of 301 million, reported the Center for Immigration Studies, a research group that advocates lower levels of immigration.

“The central question these projections raise is – what costs and benefits come with having a much larger population and a more densely settled country,” study author Steven Camarota said during a panel discussion that included both a leader in the effort to restrict immigration and a prominent advocate of expansive immigration policies.

Net annual immigration is about 1.25 million people, comprising 800,000 legal immigrants and 450,000 illegal immigrants. Camarota's projections include growth only from future immigrants, not from the 38 million immigrants now in the United States.

In the panel discussion, Roy Beck, a former environmental journalist who now heads NumbersUSA, a group that favors immigration restrictions, called the report “thoroughly depressing.”

Beck said immigration-led population growth, which took the U.S. population from 200 million in 1967 to the 300 million mark last year, is the direct result of federal policy directed by Congress.

“Illegal immigration is a big problem in this country, but it's not nearly as much of a problem as legal immigration,” Beck said.

His organization played a major role this year in mobilizing opposition to a Senate bill that would have expanded legal migration as well as providing legal status to an estimated 12 million illegal immigrants.

Demographer Ben Wattenberg, who has long hailed immigration as vital to the nation's economic and cultural vitality, dismissed anxiety about a “population explosion.”

Wattenberg said fears about immigration, centering on flows from Mexico that top both the legal and illegal immigrant categories, are a misguided reprise of earlier suspicions about Germans, Italians and Jews.

“There is always in America a nativist, anti-immigrant feeling,” he said.

But Mark Krikorian, executive director of the Center for Immigration Studies, said the study highlighted the need to inject concerns about population growth into a public debate that has focused too narrowly on immigration's implications for the nation's economy and security.

Immigration advocate Tamar Jacoby, who did not participate in the panel discussion, called high rates of immigration essential to the nation.

“Countries that are not growing are not strong countries,” said Jacoby, a senior fellow at the Manhattan Institute.

The study challenged claims that immigration can slow the aging of the U.S. population and boost the ratio of workers to retirees.

“Immigrants grow old just like everyone else,” Camarota said. He said raising the retirement age would counter the fiscal effects of an aging population.