Eighteen private schools run by the Hasidic Jewish community have been breaking the law by not providing their students with an adequate secular education, New York City officials said on Friday.

The findings were an extraordinary rebuke of the schools, known as yeshivas, which receive hundreds of millions of dollars in public money annually but have long resisted outside oversight.

The determinations about the schools, which offer intensive religious lessons in Yiddish but little instruction in English, math or other secular subjects, marked the first instance of the city concluding that private schools had failed to provide a sufficient education.

The move was all the more remarkable because it was made by a city government that has long shied away from criticizing the politically influential Hasidic community. And it stemmed from a long-stalled investigation that spanned eight years and two mayoral administrations and was often hobbled by political interference and bureaucratic inertia.

If the findings are upheld by the state education department, as is expected, the schools could be required to submit detailed improvement plans and face government monitoring. The law, however, does not make clear what consequences the schools might face if they do not commit to improving.

A spokesman for the city’s Department of Education said in a statement that it had performed a “thorough, fair review” of the Hasidic schools.

“Schools that are found to not provide a substantially equivalent education will work with the Department of Education to create and implement a remediation plan,” the spokesman, Nathaniel Styer, said. “Our goal is to educate children, not to punish the adults.”

Representatives of the schools said they had not seen the findings and could not comment. Hasidic leaders have defended the schools previously, saying they prepared students for happy and fulfilling lives in the Hasidic community.

The action by the city follows reporting by The New York Times that found that scores of all-boys Hasidic schools in Brooklyn and the lower Hudson Valley had denied their students an adequate secular education, and that teachers in some of the schools had used corporal punishment to enforce order.

The Hasidim, a fervently religious segment of the larger Orthodox Jewish community, operate more than 200 gender-segregated schools of varying quality across the state. Boys schools, in particular, provide less secular education, focusing instead on the parsing religious texts. The city investigation examined complaints about more than two dozen schools in Brooklyn that collectively enroll thousands of children.

After being informed of The Times’s reporting, Mayor Eric Adams, a longtime ally of the city’s Hasidic leaders, promised to complete the investigation into the schools, which began in 2015 under his predecessor, Bill de Blasio.

The results of the city’s investigation were summarized in a letter sent to state education officials on Friday. Of the 18 schools the city found to be deficient, officials made a final determination that four were breaking the law. The city recommended that the state make the same determination about the remaining 14. Under the law, the city has the power to make final determinations about some private schools but not others.

City officials also said five other schools they investigated were complying with the law only because of their affiliations with state-approved high school programs. Those schools will not face additional scrutiny.

Just two of the more than two dozen schools the city investigated were found to be in compliance with the law based on the quality of their instruction, echoing preliminary findings issued by the de Blasio administration late in 2019.

The city’s investigation had opened four years earlier, when a group of current and former yeshiva students and parents filed a complaint claiming that the schools were limiting children’s opportunities by not providing an adequate education.

In conducting the review, the de Blasio administration often deferred to a lawyer representing the yeshivas, The Times found, giving the schools advance notice of visits and allowing the lawyer to accompany inspectors.

Some of the schools put off the inspections for years, and city officials later acknowledged that they did not understand what they were supposed to be evaluating in the classrooms they did inspect.

Mr. de Blasio also engaged in “political horse-trading” by delaying the 2019 release of the preliminary findings on the schools, according to a report by the city’s Department of Investigation.

Both the de Blasio and Adams administrations have refused to release the details of what inspectors observed in classrooms. The nonprofit news outlet The City sued for the records, and a state court judge ruled in its favor in May. City officials have appealed the decision.

The state education commissioner, Betty Rosa, has recently increased enforcement efforts to ensure that yeshivas provide a basic education.

Last fall, Ms. Rosa overruled the city’s recommendation that a Hasidic boys’ school in Brooklyn be found in compliance with the state law. She criticized the city inquiry into that school sharply, found that the school was breaking the law and ordered it to come up with an improvement plan.

The New York State Board of Regents also passed new regulations last year, which were advanced by Ms. Rosa and laid out consequences for schools that failed to provide a basic education.

Still, those rules had been watered down after years of protest from Hasidic leaders. They were further weakened earlier this year when a judge hearing a lawsuit brought by some yeshivas ruled that the state could not close schools for being noncompliant. The state law also gives schools a lengthy timeline to show its attempts to make improvements before facing further consequences.

Mr. Adams has frequently praised the yeshivas, particularly in recent weeks as the investigation has neared its conclusion. The Department of Education conducted the investigation using its own staff members, but as mayor, Mr. Adams controls the department.

“Instead of us focusing on how do we duplicate the success of improving our children, we attack the yeshivas that are providing a quality education that is embracing our children,” Mr. Adams said in May while addressing a crowd of yeshiva leaders.

During a June visit with Hasidic leaders, he offered another strong defense of the schools.

“It’s unfortunate that those outside your community don’t understand that all you want to do is live in peace, educate your children and be able to provide for your community,” Mr. Adams said. “I know that because, as I stated, I’m not a new friend, I’m an old friend, and old friends respect each other.”

Then he accepted a plaque from Hasidic leaders thanking him for protecting yeshivas.