SAN FRANCISCO, Oct 6: The post-9/11 security measures have had a far greater impact on legal immigration to the United States than on illegal entry, according to a study by the Pew Hispanic Center.

Though the level of immigration to the United States has subsided by about 25 per cent from a peak of 1.5 million a year in 1999 and 2000, more annual immigration is now illegal than legal, the study found.

The study calculated that the number of legal permanent residents entering the United States declined to 455,000 last year from 647,000 at the peak in 2000.

“There seems to have been a tradeoff here between legal and illegal immigration,” said Jeffrey S. Passel, a senior researcher at the Pew Hispanic Center. “Basically, we got about 1.1 to 1.2 million throughout the ‘90s and post-2002. We’re getting roughly the same number, but the legals went down.” The proportion of authorized immigrants went from more than half to 4 out of 10, he said.