Mayor Zohran Mamdani plans to sue the New York City Council, arguing that its bill granting hefty bonuses to some of the city’s lowest-paid school workers violates collective bargaining law, a city spokesman said.

The decision to sue comes the same day that the City Council bill became law, after the mayor declined to issue a veto, knowing that the Council had the votes to override it.

The suit, expected to be filed in State Supreme Court in Manhattan as early as Monday, could set the scene for the mayor’s highest-profile fight so far with a public sector union and stands to embitter his already fraught relationship with the Council’s speaker, Julie Menin, who championed the legislation.

But a spokesman for the mayor said the city was acting to protect labor rights.

“We will not allow the political process to replace the collective bargaining table,” said the spokesman, Matt Rauschenbach. “The city is filing a lawsuit to protect the right of every union to negotiate on behalf of its members, to fight for the workplace they deserve and ensure that workers — not politicians — determine their own futures.”

The Council bill, backed by the influential United Federation of Teachers, would grant $10,000 bonuses to New York City teaching assistants, who often work one-on-one with children with disabilities and are among the worst-paid workers in the city’s public school system.

Mr. Mamdani, a democratic socialist who often closely aligns himself with organized labor, supported the bonuses on the campaign trail, but came out against them once he became mayor. He argued that the bill violated state and city labor law, which requires such issues to be hammered out via collective bargaining.

The teachers’ union backed Mr. Mamdani for mayor only after he won the Democratic primary last summer.

Mr. Rauschenbach said the administration was committed to raising paraprofessional pay by other means.

“We want to engage in productive conversations with the U.F.T. to permanently address this inequity,” he said.

The mayor, who campaigned on the promise of making New York more affordable for all, now finds himself in the politically awkward position of fighting the teachers’ union and potentially blocking bonuses for low-income city employees who work with vulnerable children.

Nor is Ms. Menin’s position without political complications. The speaker often casts herself as a more fiscally moderate foil to the mayor, but the legislation she has championed would, according to critics, amount to an expensive end-run around collective bargaining that other unions would be eager to emulate.

As evidence, opponents of the legislation note that Joann Ariola, the councilwoman who chairs the Committee on Fire and Emergency Management, recently pointed to the paraprofessional bonuses in her request to raise the pay for the Fire Department’s emergency medical technicians, some of whom make as little as $18 an hour.

Ms. Menin had no immediate comment on Wednesday, but she has argued previously that the bill is carefully constructed to avoid violating state labor law. In an interview on Wednesday morning on “The Brian Lehrer Show” on WNYC, Ms. Menin said that it made fiscal sense to fund the bonuses and help fill paraprofessional job vacancies, at a time when the city is spending more than $1 billion a year to send children with disabilities to private schools where they can receive more specialized services.

The first tranche of the bonuses is not due until Jan. 1.

The ambiguity surrounding the legislation’s future did not stop the teachers’ union from declaring victory.

In an email to paraprofessionals on Wednesday, Michael Mulgrew, the president of the union, wrote, “Today, the RESPECT check became law! All of our hard work has gotten us to this moment. As of today, paraprofessionals can expect to receive their first RESPECT payment in January.”

But Mr. Mulgrew, who has threatened “all-out war” on the mayor if Mr. Mamdani tries to block the legislation, also acknowledged that uncertainties remained.

“The city could challenge this legislation in court,” he said. “But we are prepared for this.”

The roughly 26,000 paraprofessionals who work in New York’s public school system, America’s largest, have a starting salary of about $32,000 a year.

The legislation would cost an estimated $325 million, according to the nonpartisan Citizens Budget Commission, a watchdog group.

The commission also warned that the law threatened to sow havoc in the city’s half-century-old collective bargaining practices, by establishing a precedent that when unions did not get what they wanted at the bargaining table, they could just turn to the City Council for help.