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 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 state Education Department upholds the findings, as is expected, the schools could be required to submit detailed improvement plans and undergo 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 the agency had performed a “thorough, fair review” of the Hasidic schools.

“As always, our goal is to build trust, work with the community, and ensure schools are in compliance with state education law and regulations,” the spokesman, Nathaniel Styer, said, adding, “Our goal is to educate children, not to punish the adults.”

A spokesman for some of the yeshivas, Richard Bamberger, said in a statement that the Hasidic community “rejects the attempt to measure the efficacy of yeshiva education by applying a skewed set of technical requirements.”

“Utilizing a government checklist devised and enforced by lawyers may help explain the state of public education,” he said. “It is designed to obscure rather than illuminate the beauty and success of yeshiva education.”

Advocates for yeshiva reform said they were cautiously optimistic about what the findings would yield.

“We hope that the completion of this investigation compels the city and Mayor Eric Adams to act on behalf of thousands of students who are being deprived of their right to a sound basic education,” said Beatrice Weber, executive director of Yaffed, the group of former students and parents whose complaints gave rise to the investigation.

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 than girls schools do, focusing instead on the parsing of 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 letters 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. A spokesman for the state Education Department said officials there were reviewing the city’s recommendations.

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. One of the schools was a yeshiva for girls.

In the letters summarizing the investigation, officials described visiting schools and finding deficiencies in course planning or proof of teacher training. In some cases, officials reported seeing no instruction at all in core subjects.

After multiple visits to Oholei Torah in Crown Heights, one of the largest yeshivas in the state, inspectors said they had found “insufficient evidence that teachers have the appropriate knowledge, skill and disposition to deliver” adequate secular instruction.

At another school, Bnei Shimon Yisroel of Sopron in Williamsburg, inspectors noted a complete lack of English-language instruction in reading, spelling, writing, math, geography, history, civics and science.

Representatives of those schools did not respond to messages seeking comment.

On Friday, the former yeshiva student who founded Yaffed, Naftuli Moster, said he felt gratified by the findings.

“Ten years of my adult life have been spent attempting to recover what I was never provided in my childhood,” Mr. Moster said. “This report is vindication of our efforts, and offers a promise to children today that their inadequate education will not go unnoticed, and may yet improve.”

Over the past eight years, the city investigation has faced a number of hurdles. 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. 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 administrators.

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.

Alex Lemonides contributed reporting.