A deep recession and tougher border enforcement have led to a sharp decline in the number of immigrants entering the United States illegally in the past five years, contributing to the first significant reversal in the growth of their numbers in two decades, according to a new report by the Pew Hispanic Center.

The number of illegal immigrants entering the United States plunged by almost two-thirds between 2005 and 2009, a dramatic shift after years of growth in the population, according to the report.

In the first half of the decade, an average of 850,000 people a year entered the United States without authorization, according to the report, released Wednesday. As the economy plunged into recession between 2007 and 2009, that number fell to 300,000.

The drop has contributed to an 8 percent decrease in the estimated number of illegal immigrants living in the United States, from a peak of 12 million in 2007 to 11.1 million in 2009, the report said. Of the 11.1 million, 60 percent came from Mexico, 20 percent from other parts of Latin America, 11 percent from Asia, and 9 percent from Africa, Europe, Canada and elsewhere. The new figures come amid a heated national debate over efforts by Arizona and other jurisdictions to identify people who are here illegally and push to have them deported.

Virginia, Florida and Nevada had the most significant declines in their populations of illegal immigrants.The large declines in Florida and Nevada were probably due to the housing crisis and the loss of thousands of construction jobs, which immigrants often fill, experts said. Florida's illegal immigrant population fell by 375,000, to an estimated 675,000, between 2008 and 2009, and Nevada's decreased by 50,000 to an estimated 180,000 during that period.

The number in Virginia fell by 65,000, to 240,000, a decline that Jeffrey Passel, an author of the report, attributed to the economy as well as to stricter legislation passed in Prince William County in 2007 and 2008.

Although the report's authors did not have information about population decreases at the county level, a 2009 University of Virginia study of the legislation's effect on Prince William showed a decline in the number of children enrolled in ESOL classes and in the number of uninsured mothers giving birth, both suggestive of the presence of recent and poor immigrants, said Audrey Singer, who specializes in immigration at the Brookings Institution.

Corey A. Stewart, chairman of the Prince William Board of County Supervisors and a champion of the legislation, said he was "proud" of the county's role in decreasing the illegal immigrant population. "Our resolution on illegal immigration resulted in thousands of illegal aliens leaving Prince William," said Stewart, who has called for Virginia to adopt a law similar to Arizona's.

But Stewart (R-At Large) predicted that the declining numbers of illegal immigrants in Virginia would be temporary without stricter enforcement statewide. "They're just going to come back once the economy recovers," he said.

Douglas Massey, a Princeton University sociologist who studies migration, said the recession and the lack of jobs are major factors in the reduction of people entering the country illegally.

The unemployment rate for unauthorized immigrants is 10.4 percent, higher than that of either U.S.-born residents or legal immigrants, said the Pew report, which used data from the Current Population Survey, a monthly report on 55,000 households conducted by the U.S. Bureau of Labor Statistics and the Census Bureau.Pew, a premier research organization on immigration matters, calculated the number of unauthorized immigrants by adjusting the statistics up by 10 to 15 percent to account for undercounts, said D'Vera Cohn, an author of the report. The size of the undercount was determined by factors including Mexico's population size, U.S. mortality data and research conducted during the 2000 Census around the issue of undercounting, she said.

Massey also pointed to a rise in guest-worker visas, from 104,000 in 2000 to 302,000 in 2009, allowing more immigrants to come to the United States legally. The other likely reasons for the decline, he said, include an increase in law enforcement and deportations, and enactment of stricter legislation against illegal immigrants.