Granting legal status to 11 million undocumented foreigners living in the United States would significantly add to government budget deficits, according to a study released on Monday by the conservative Heritage Foundation.

Heritage, a think tank headed by former Senator Jim DeMint, has assumed a leading role in opposition to a bipartisan, comprehensive immigration bill being considered by the U.S. Senate.

It is only the first of what is expected to be a flurry of competing studies attempting to influence the upcoming immigration debate.

The legislation couples the legalization proposal with a new program to admit high-skilled and low-skilled workers to fill jobs that industry and agriculture say are going begging because of a shortage of available U.S. workers. The Heritage study did not look at the impact on the economy of those provisions.

Coming three days before the Senate Judiciary Committee is scheduled to begin the immigration measure, the Heritage Foundation study estimated that legalizing the 11 million would put a severe strain on government programs.

Over the next 50 years, it would cost taxpayers a net $6.3 trillion, according DeMint, a former Republican senator from South Carolina and a favorite of the conservative Tea Party movement. That would work out to $124 billion a year on average.

The study said that over the course of their lives, these undocumented immigrants-turned-citizens would receive $9.4 trillion in government benefits and services and pay only $3.1 trillion in taxes.

The government benefits and services include healthcare, Social Security, public education, aid to low-income families and a range of other programs.

The Heritage Foundation report was blasted by other conservatives, even before it was issued.

Cato Institute, in a website posting over the weekend, said that the Heritage Foundation study was an update of a "fatally flawed" analysis it issued in 2007.

The earlier findings, according to the Cato Institute, failed to take into account the potential economic benefits of integrating 11 million illegal residents into society.

It specifically mentioned the likelihood of growing tax revenues and economic expansion as a result of broadening the legal U.S. workforce and population.

Following the 2012 elections in which around 70 percent of Hispanic-American voters supported the re-election of Democratic President Barack Obama, Republicans began re-examining their opposition to immigration reforms, including providing a "pathway to citizenship" for the 11 million.

Nonetheless, there are deep splits within the Republican Party on exactly how to address immigration problems and whether the main focus instead ought to be on enhanced border security and issuing more visas to lure highly skilled foreigners to work in the United States.

Grover Norquist, a leading anti-tax activist influential in Republican circles, has joined in supporting the Senate's bipartisan immigration bill, testifying in favor of it last month before the Judiciary panel.

Norquist has argued that the measure will boost economic growth, as has Douglas Holtz-Eakin, a former aide to Republican President George W. Bush.

(Reporting by Thomas Ferraro; Additional reporting by Richard Cowan; Editing by Fred Barbash and Cynthia Osterman)