WASHINGTON (Reuters) -- Illegal immigrants to the United States cost the federal government over $10 billion a year, but that figure would increase almost threefold if they were granted legal status, according to a study released on Wednesday.

The study by the Center for Immigration Studies, a Washington think-tank which advocates slowing legal immigration and clamping down on illegal migration, measured the amount of revenue illegal immigrants provide through taxes against the government services they and their families consume.

There are an estimated 10 million illegal aliens in the United States.

"Households headed by illegal aliens imposed more than $26.3 billion in costs on the federal government in 2002 and paid only $16 billion in taxes, creating a net fiscal deficit of $10.4 billion, or $2,700 per illegal household," Steve Camarota, author of the report.

These costs, derived from Census Bureau figures, include Medicaid, which provides medical insurance for the poor, other emergency medical treatment, food assistance programs and the federal prison system, where roughly 17 percent of inmates are illegal immigrants.

They did not include costs to local and state governments which would push the deficit much higher, Camarota said.

Earlier this year, President Bush proposed a guest worker program that would allow some of the nation's illegal aliens to acquire legal guest worker status. Democratic presidential candidate John Kerry has proposed granting an amnesty for most illegal immigrants who have been in the United States for at least five years.

GREATER COSTS

But the report said legalizing illegal aliens would vastly increase their costs. More would pay taxes but because the majority are poor and hold low-paying jobs, this would be vastly outweighed by the increased government welfare services they would consume.

Camarota said the only effective way to lighten the fiscal burden was to enforce immigration laws by stepping up border patrols and reducing the number of illegal aliens in the country. At the moment, laws making it illegal to hire illegal immigrants are almost totally ignored.

Many employers would likely fight such steps because they rely on illegal migrants as a source of low paid labor.

Immigration advocates criticized the report for ignoring the human side of the equation and failing to suggest any constructive solutions.

"Our legal immigration system is broken. Family members have to wait years to gain legal entry to this country to be reunited. It can take 10 years to get a legal visa for a spouse," said Michele Waslin, a senior analyst for the National Council of La Raza, a major Hispanic organization.

"We need comprehensive legal immigration, which includes legalizing undocumented immigrants who can prove they are law abiding and pay their taxes," Waslin said.

Georgetown University immigration expert Lindsay Lowell said U.S. immigration law had helped create an underprivileged underclass of low income workers that helped employers but imposed substantial costs on the rest of society.