WASHINGTON -- Comprehensive immigration legislation passed by the Senate would reduce illegal immigration into the U.S. by one-third to one-half beyond what would happen under existing law, the Congressional Budget Office said Wednesday.

That's a significantly greater reduction than the budget office said would have resulted from an earlier version of the bill approved by the Senate Judiciary Committee, which would have cut illegal immigration only by 25 per cent.

Partly in response to that earlier finding, senators agreed to greatly boost border security in the bill and take steps against people who overstay their visas. Those changes helped the legislation pass the Democratic-controlled Senate with a bipartisan 68-32 majority last week. It's now pending in the Republican-led House, where it faces an uncertain future.

The CBO also said that despite spending $36 billion more on border security than would have happened under earlier versions of the bill, the legislation would still reduce the budget deficit by $158 billion over 10 years and $685 billion in the decade after that. Taxes paid by newly legalized residents would outpace new spending for government benefits and other costs under the bill.

The legislation would double border patrol agents stationed along the U.S.-Mexico border while calling for hundreds of miles of fencing and requiring all businesses to check their workers' legal status. Some 11 million immigrants already here illegally would be able to attain citizenship over 13 years, if they pay fines and taxes and meet certain requirements. New and expanded worker visa programs would allow tens of thousands of new workers into the country for high-skilled and low-skilled professions.

Under the bill as passed, the report found, there would be around 2.4 million fewer immigrants coming illegally into the country or illegally overstaying their visas over the next 10 years than would happen under current law.