The number of illegal immigrants in the United States has grown to as many as 12 million, and they now account for about one in every 20 workers, a new estimate says.

Efforts to curb illegal immigration have not slowed the pace, said a report Tuesday by the Pew Hispanic Center.

Instead, the report's author said, those efforts are having an unintended consequence: People who illegally enter the United States from Mexico are staying longer because it is harder to move back and forth across the border.

"The security has done more to keep people from going back to Mexico than it has to keep them from coming in," said Jeffrey Passel, a senior research associate at the center.

It is difficult to accurately measure the number of illegal immigrants in the United States, but most public agencies and private groups had settled on a figure of about 11 million.

The Pew Hispanic Center used Census Bureau data to estimate that the United States had 11.1 million illegal immigrants in March 2005. The center used monthly population estimates to project a current total of 11.5 million to 12 million.

The report estimates that 850,000 illegal immigrants have arrived in United States each year since 2000.

President Bush has called for a program that would grant temporary worker status to illegal immigrants already here. The House rejected the program and instead passed a border security bill last year that leaned toward lawmakers who were calling for a crackdown.

The Senate is trying to address both border security and the temporary worker program, but consensus has been elusive. Senate Judiciary Committee Chairman Arlen Specter, R-Pa., has said he hopes his panel will produce a bill by the end of March.

There are about 7.2 million undocumented workers in the U.S., or about 5 percent of the country's work force, the Pew report said.

It estimated that illegal immigrants fill a quarter of all agricultural jobs, 17 percent of office and house cleaning positions, 14 percent of construction jobs and 12 percent in food preparation.

"Especially if we look at the Mexicans, these are people with fairly low levels of formal education," Passel said. "They're not able to get licensing or credentials in the United States because of their status, so the kinds of jobs available to them in the United States are somewhat limited."

Business leaders and advocates for immigrants' rights argue that America's economy would collapse if all the illegal workers were deported.

"Undocumented immigrants do pay taxes, and they do contribute to the economic, social and cultural developments of their communities," said Peta Ikambana of the American Friends Service Committee. The group was organizing a rally near the Capitol on Tuesday to protest the House bill.

"Just building walls will not stop immigration," Ikambana said. "Those that are here will just go underground."

Steven Camarota of the Center for Immigration Studies, which advocates tougher border enforcement, said he isn't surprised that the number of illegal immigrants continues to climb. He called the government's crackdown halfhearted at best.

Camarota pointed to a recent government report showing that very few businesses are fined for hiring illegal immigrants. The government filed only three notices that it intended to fine companies in 2004, down from 417 notices in 1999, according to a report by the Government Accountability Office.

Camarota said there would be plenty of Americans willing to accept jobs done by illegal immigrants if they paid adequate wages and benefits.

Tuesday's report by the Pew Hispanic Center said Mexicans make up 56 percent of illegal immigrants. An additional 22 percent come from other Latin American countries, mainly in Central America. About 13 percent are from Asia, and Europe and Canada combine for 6 percent.