City correction brass violated the Big Apple’s controversial “sanctuary city” rules by allowing a department investigator to tip off the feds about illegal immigrant inmates, a scathing new report said.

The Department of Correction dropped the ball when it let the staffer, who was not identified, tell US Immigration and Customs Enforcement officials that illegal immigrants were being released, allowing the feds to nab them for deportation, a Department of Investigation report said Thursday.

The move — which officials said was done “unwittingly” — violated city law that bars cooperation with ICE on civil deportation cases.

“New York City law and DOC policy do not allow city resources to be used for the purpose of facilitating the enforcement of federal immigration law, and that prohibition includes the sharing of information with our federal enforcement partners for that purpose,” DOI Commissioner Jocelyn Strauber said.

“DOC found that in at least two instances a DOC investigator unwittingly violated the law and DOC policy and that DOC failed to provide proper guidance and training,” Strauber said.

The report says the officer inappropriately shared details about inmate release dates with federal immigration officials, which the report put on DOC for not providing proper training on sharing information with the feds.

According to the 68-page report, the correction investigator, assigned to a joint federal task force, first told the feds in November that illegal immigrant Pedro Mujica Villa Nueva, who was charged with assault and grand larceny in Queens, was being held at Rikers Island.

In December, the investigator provided Homeland Security Investigations with a screen shot of Villa Nueva’s correction report with his photo, court information and other details, the report said.

The report said that on Feb. 5, the investigator “assisted” federal agents take migrant Cristian Concepcion, who was in the country illegally, into custody.

Concepcion was convicted of third-degree assault and was due to be released, the report said.

The report did not recommend disciplinary action against the investigator or DOC, only several corrective measures.

In a response to the report, city DOC officials generally agreed with the DOI’s recommendations and said many had already begun to be implemented, including re-enforcing sanctuary city requirements with correction staffers and setting up protocols to deal with federal immigration authorities.

“The department has already taken affirmative and concrete steps to implement this recommendation,” officials said in response to the additional training suggestion.

“The department will continue to explore opportunities for training personnel, including incorporating these modules into recruit and promotional academy curricula.”

In a statement Thursday, officials at City Hall said the correction employee involved has undergone additional training. 

“As Mayor [Eric] Adams has repeatedly stated, New York City does not — and will not — participate in civil immigration enforcement, in accordance with local law,” the statement said. 

“To be clear, no one at DOC or within New York City government — aside from the employee himself — had any role in or knowledge of the sharing of sensitive information with federal officials.”