A federal judge has told the City Council that it has to redraw the map of the political boundaries of its nine district seats that councillors had approved last fall after sparring over its outlines for months.

Opponents of the new layout, which sliced up Dorchester-based District 3, particularly the Neponset and Adams Village neighborhoods, then filed a lawsuit, arguing that race had wrongly played a predominant role in the redistricting, as the redrawing process is known, and on Monday, US Judge Patti Saris granted the plaintiffs a preliminary injunction, ruling that they had “demonstrated a likelihood of success” with their filing.

“The ball is back in the City Council’s court,” she wrote.

The decision also blocks the city of Boston from using the map in this fall’s elections. It comes as council candidates are out gathering voter signatures and incumbents are in the thick of hearings on the annual budget. The judge’s order throws into disarray the municipal electoral process, leaving a lack of clarity as to whether city officials can use the map passed in 2012 since the enacted map from last fall cannot be used.

Late on Tuesday , City Hall officials said they are reviewing options for extending certain filing deadlines for candidates.

Councillors redraw the political boundaries every ten years in a typically contentious process. This past cycle, councillors were faced with organizing a map that accounted for a population boom in South Boston-based District 2, and a population loss in District 3.

The city could seek to delay the elections, as happened in 1983, when district seats were first introduced. That year’s map spurred a legal challenge from the late Mel King, and a court order forced councillors back to the drawing board. The preliminary election was delayed two weeks and the general election was pushed back a week.

“In my view, the City Council is best positioned to redraw the lines in light of traditional districting principles and the Constitution,” Saris wrote in her 43-page decision that was released Monday afternoon. “The role of race in redistricting is complicated and in flux, and the Court finds that the City Council acted in good faith in trying to comply with complex voting rights laws.”

The opponents who pressed for the preliminary injunction included Maureen Feeney, the former city clerk and former District 3 councillor; the St. Vincent’s Lower End Neighborhood Association; and Rasheed Walters, a District 4 resident who has written for the Boston Herald.

The named defendants in the lawsuit include Mayor Wu and the City Council, even though the four councillors who voted against the enacted map have supported the efforts to overturn it. A Wu spokesperson said on Monday night that the administration is reviewing the decision.

District 2 City Councillor Ed Flynn, District 3 Councillor Frank Baker, and City Councillor At-Large Michael Flaherty have funneled money toward lawyers waging the battle in court against the map; Councillor At-Large Erin Murphy took the stand to testify in favor of an overturn as did Flaherty and Flynn.

“I am pleased with the ruling because it supports my long-held belief that this map unfairly robbed District 3 and the citizens of Boston of its voice and was designed to weaken its position in Boston politics,” Baker said in a statement. “Gerrymandering is gerrymandering – whether in pursuit of progressive or conservative goals.”

The tossed-out unified the Vietnamese community in Fields Corner, also known as “Little Saigon,” by moving precincts 16-1 and 16-3 from District 4 to District 3; it also unified the Roslindale neighborhood by shifting two precincts to District 5 from District 4. (Mayor Wu backed both moves.) The map also disrupted the electoral scene at the Anne Lynch public housing development by moving some residents to District 3 from District 2.

But the loudest protests against what the council did last fall came from Dorchester about the splitting of Adams Corner/Adams Village, an area that includes white super-voters.

Saris found in her ruling “a likelihood of success on the claim that race predominated the City Council’s decision to move four precincts with largely White super-voters from District 3 to District 4.”

“Judge Saris’s ruling today is a victory for transparency, accountability, and the people of the city of Boston,” Murphy said in a statement. “The United States District Court identified a deeply flawed process, and I welcome the opportunity to join my colleagues in rewriting more equitable districts that protect our constituents’ Constitutional rights.”

The enacted map also flipped Fields Corner schoolteacher and pastor Joel Richards from District 4 to District 3. Richards, who lives in Ward 16-Precinct 1 and ran for the District 4 seat in 2021, launched his campaign for District 3 in December. The race is now wide open after Baker announced he wouldn’t seek another two-year term and other candidates are piling in.

Richards said the judge’s decision doesn’t change anything for him; he plans to keep campaigning in District 3.

“Every new map that came out had me in District 3,” he said of last fall’s process. “I don’t see anything different for me.”