A repeat sex offender who is so dangerous that a state psychologist believes he will likely strike again is about to be set free over the objections of the judge in his case because of a little-noticed state high court ruling.

Jerome Arps — who raped a woman on the steps of the Cathedral of St. John the Divine in 2006 — had been locked up under rules that allowed sex fiends to be jailed after their release date if they are a danger to the public.

But a 2014 ruling by the state Court of Appeals has changed those rules. Now, pervs who serve their maximum jail time can be held only if they have a mental problem that is more serious than just a personality disorder.

Arps, 58, doesn’t meet this standard — so he technically qualifies for release.

Experts, however, fear that he’s still a danger to the public and that it will only be a matter of time before he rapes again if he is freed.

“Mr. Arp’s psychiatric condition makes him strongly predisposed to commit future sex offenses,” state psychiatrist Dr. Frances Charder said in court papers, diagnosing him as a psychopath with antisocial personality disorder.

“Mr. Arp’s ASPD along with his very high level of psycopathy have combined to produce a predisposition to gratify sexual urges in an illegal and deviant manner, without regard for the rights of others,” she said.

But Charder can’t prove he lacks the control to keep himself from committing sexual assaults, Manhattan Supreme Court Justice Daniel Conviser found.

In the Sept. 8 decision to release Arps, Conviser wrote that the Court of Appeals imposed “rigorous new standards” that “may be impossible to meet in most . . . cases.”

Conviser agreed that Arps — whose offenses date back to when he helped gang-rape a 14-year-old when he was a minor — was still dangerous.

“Indeed, at the age of 50, when the vast majority of sexual offenders have already aged out of their offending behaviors, he committed rape,” Conviser said.

The appeals court change will affect about 300 offenders. The burden is now on the state to prove they have mental abnormalities, not just personality disorders.