Washington:  U.S. and Mexican officials signed a controversial agreement last week that could allow millions of legal and undocumented Mexican immigrants who work in the United States to collect U.S. Social Security benefits.

Congress and the Mexican Senate must approve the agreement, but U.S. lawmakers have routinely approved similar agreements with 20 other countries in the past.

Under the agreement, Mexican workers who have divided their working lives between the United States and Mexico would be eligible for partial U.S. or Mexican retirement benefits based on combined credits earned from both countries. U.S. citizens working in Mexico also would qualify.

To qualify for U.S. Social Security benefits, Mexicans must prove they have worked in the United States at least 18 months. Payments are made on a prorated basis, depending on years worked.

Social Security Administration officials estimate 50,000 U.S. and Mexican workers may be eligible for retirement benefits during the first five years of the agreement.

But last year, the General Accounting Office, the investigative arm of Congress, questioned the accuracy of the projections, saying Social Security officials had failed to account for the presence of many potentially eligible, undocumented Mexican immigrants.

Census figures show the United States is home to 9 million Mexican citizens. About 5 million are here illegally, the government estimates.

Critics say the agreement could allow thousands of undocumented Mexican immigrants to collect U.S. retirement benefits.

''Until we straighten out the mess in our immigration system -- not to mention our Social Security system -- these sorts of agreements are premature and fiscally irresponsible,'' said Dan Stein, executive director of the Federation for American Immigration Reform, a group that advocates stricter immigration policies.

Social Security officials counter that undocumented immigrants are not eligible for U.S. benefits. ''A totalization agreement doesn't change our immigration laws,'' said Carolyn Cheezum, a spokeswoman with the Social Security Administration.

But Cheezum acknowledged that undocumented immigrants who become legalized residents could receive credit for prior earnings if they prove they paid into the U.S. system.