The city teachers union filed legal papers Tuesday to fight the lawsuit aimed at loosening tenure protections for incompetent teachers.

The United Federation of Teachers filed a motion in Staten Island Supreme Court to intervene as defendants in the case.

Students and parent plaintiffs claim New York’s current tenure law is unconstitutional for denying the neediest kids a “sound, basic education” by saddling them with bad teachers.

UFT president Mike Mulgrew said the union should be allowed to join the case to defend tenure rights that protect teachers.