WASHINGTON – 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.

Tuesday’s report by the Pew Hispanic Center said Mexicans make up 56 percent of illegal immigrants.