Faculty and staff at the City University of New York are suing the school over its COVID-19 vaccine mandate — claiming the policy isn’t fair because it doesn’t apply to all employees.

The union representing CUNY’s professors said it was forced to sue after urging the administration “for months” to expand the vaccine requirement, a spokesperson told The Post.

TheProfessional Staff Congress escalated its complaint last week by filing suit in Manhattan Supreme Court to challenge the legality of the policy — which applies to its 25,000 active members but not more than 10,000 other school staffers.

“The PSC strongly supports vaccine mandates as a public health necessity, but CUNY’s is a fake mandate that subjects our members to a blatant double standard and leaves our campuses no safer than without it,” said James Davis, president of the union.

While custodians, campus security workers and cafeteria staffers who decline to get vaccinated can keep their jobs, other CUNY employees without medical or religious exemptions may face discipline up to and including their termination, court documents suggest.

“We have taken this legal action not in defense of vaccine objectors but on behalf of all of the nearly 25,000 active CUNY employees that we represent,” Davis wrote in a memo to members.

The lawsuit called the vaccine policy “arbitrary and capricious.”

“There is no rational basis to require vaccination from some employees in a facility but not other employees in that same facility, many of whom work in direct proximity to each other and interact with the University community in the same way,” read the complaint.

Gov. Kathy Hochul ordered over winter break that all faculty at CUNY and the State University of New York get vaccinated against COVID-19.

The union put in an immediate request to CUNY to bargain how that mandate would be implemented, according to a communication to members obtained by The Post.

The vaccine rule, which was planned to go into effect on April 1, was delayed to May 27, separate memos showed.

A spokesperson for the PSC said the mandate was pushed back “by mutual agreement” to continue negotiations. That process is ongoing as both parties hammer out discipline measures for workers who do not comply.

CUNY employees refusing to get the jab were required to request a medical or religious exemption last month. The workers whose requests get denied have a grace period of six weeks to get vaccinated to comply with the mandate, the union told members.

The lawsuit comes as the governor appears posed to loosen COVID-19 protocols for state workers, according to a memo from the Office of Employee Relations announcing it would lift the testing requirement for unvaccinated employees starting Tuesday, The Times Union reported.

A spokesperson for CUNY said Tuesday it had not been served with a formal complaint and declined to comment on pending litigation.