Updated 4 p.m. — The United Federation of Teachers is officially jumping into a legal battle against advocates challenging New York’s teacher job protection laws.

The union announced on Tuesday that it would file a motion to intervene in a lawsuit that seeks to scale back tenure, which the parent advocates say makes it too hard to fire ineffective teachers. Though the lawsuit is filed against the city and state education departments, the union is arguing it should be permitted to argue on behalf of its 100,000 members, since they would be affected by the judge’s decision.

The filing focuses on the union’s reasoning for stepping into the case, which UFT President Michael Mulgrew wrote in an affidavit attempts to “undermine the quality of education in New York.” It does not go into a legal defense of teacher tenure, but outlines its general merits.

The lawsuit also pits the union against a former ally, Mona Davids, who is among the parents suing to undo the tenure laws. Davids heads the New York City Parents Union, which consulted with the UFT on a union-sponsored parent advocacy group two years ago.

“The union’s interests are not necessarily in the best interests of the kids,” said Davids, who said she worked with the UFT because she wanted parents to be more involved in legislative affairs. “I don’t have to agree with them on everything.”

Sam Pirozzolo, another of the parent plaintiffs, said the UFT’s involvement will have little effect.

“The UFT is not a part of this case,” Pirozzolo said. “By the UFT sticking their nose into parents’ business, it once again shows that the only thing they care about is themselves and their membership.”

Davids’ group is one of two that is arguing that more than three years of experience should be needed for teachers to receive lifelong job protections and that it’s too hard to remove ineffective teachers once they’ve been granted tenure. Former CNN anchor Campbell Brown, who has lobbied for more flexible teacher firing rules for years, is planning to file a separate but similar lawsuit in the coming weeks.

A copy of the 14-page legal motion is below: