Public Advocate Jumaane Williams has vowed to invoke an untested power under the city charter to block Mayor Bill de Blasio from collecting property taxes if he doesn’t freeze NYPD hiring and take cops out of schools.

“It hasn’t been done before, but that’s not a good reason not to do it now,” Williams said during a remote press briefing Tuesday morning.

Williams made the 11th-hour ultimatum just ahead of a City Council vote expected late Tuesday on a $87 billion budget deal that would sever $1 billion from the Police Department’s $6 billion budget. It’s unclear if the move would hold up to a legal challenge.

Williams joins a handful of council members who feel the budget doesn’t go far enough to cut NYPD spending following nationwide calls for police reform sparked by the death of George Floyd. Several other local lawmakers plan to vote against the budget because it takes too many resources away from cops at a time when violence is surging in the city.

Williams’ demands include keeping the NYPD’s workforce at its current level of 36,000 officers.

“By just saying we’re hiring additional police officers I think is sending the wrong message,” he said.

Williams is also demanding a “restorative justice model” of public safety in city schools instead of using school safety officers to enforce the law. The current proposed budget already moves the school safety officers out of the NYPD and into the Department of Education’s jurisdiction.

Williams said the section of the charter that requires the public advocate to sign off on tax warrants is usually “done pro forma,” but he has the authority to withhold his signature thus blocking the city from collecting a major source of revenue during a coronavirus-induced budget crisis.