In a significant reversal, the Bloomberg administration said Friday that it would support a City Council bill that would hamper federal authorities’ ability to detain, and eventually deport, foreign-born inmates on Rikers Island who are about to be released.

The decision is an important victory for the Council speaker, Christine C. Quinn, the sponsor of the bill, which is now almost certain to become law, and for immigrant advocates, who have long assailed the city’s cooperation with immigration agents based at the prison.

Corrections Department officials routinely share lists of foreign-born inmates with immigration authorities, who then take custody of, detain and deport thousands of people who had been charged with misdemeanors and felonies. The arrangement is common across the country.

The bill would not end the practice, known as the criminal detainer program, in New York City. But it would prevent corrections officials from transferring inmates to federal custody, even immigrants in theUnited Statesillegally, if prosecutors declined to press charges against them, and if they had no convictions or outstanding warrants, had not previously been ordered deported and did not show up on the terrorist watch list.

As a result, the immigrants would be released if they were not defendants in criminal cases, regardless of whether federal officials wanted them deported.

“The criminal detainer program had become the immigrant dragnet program,” Ms. Quinn said. “We don’t support that.”

Ms. Quinn, a likely mayoral candidate in 2013, said the bill could keep hundreds of people, perhaps as many as 1,000, from being deported every year.

In the past, Mayor Michael R. Bloomberg and his advisers have defended the city’s cooperation with immigration officials as a matter of public safety. But after extensive negotiations with Ms. Quinn’s office, the administration decided to support the bill.

“Our goal is always to protect public safety and maintain national security, while ensuringNew York remains the most immigrant-friendly city in the nation,” said John Feinblatt, the mayor’s chief policy adviser. “This strikes the right balance.”

Luis Martinez, a spokesman for Immigration and Customs Enforcement, declined to comment on the legislation.

Mr. Bloomberg’s decision comes as the Obama administration has placed a priority on deporting noncitizen criminals who pose a threat to the public, while focusing less on illegal immigrants who do not pose a threat.

Supporters of more restrictive immigration laws have criticized the Council bill as a get-out-of-jail-free card for illegal immigrants. Jessica Vaughan of the Center for Immigration Studies inWashington has said it amounts to “playing Russian roulette with public safety.”

Councilwoman Melissa Mark-Viverito, a Democrat who represents East Harlemand a co-sponsor of the bill, dismissed that charge, saying the measure would affect only “people who do not pose a risk to safety and security.”

Ms. Mark-Viverito also said that curtailing the program would save the city a significant amount of money, possibly tens of millions of dollars.

Ms. Quinn and Ms. Mark-Viverito had planned a rally on Sunday at an Upper Manhattan church to draw support for the legislation. The rally now will most likely be more of a celebration, Ms. Quinn’s office said.

The City Council will hold a hearing on the issue on Monday, when Robert M. Morgenthau, the former longtime Manhattan district attorney, is expected to testify in support of the bill. The Council is expected to vote on it before the end of the year.

Advocates for immigrants hailed the mayor’s decision as an important step toward protecting the rights of foreign-born New Yorkers.

“There’s really an evolving consensus about the corrosive impact of an aggressive deportation strategy,” said Andrew Friedman of Make the Road New York. “This is a clear statement that it is bad for New York in so many ways to facilitate the deportation of New Yorkers.”

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