UFT boss Michael Mulgrew threatens to create a $600 million deficit in NYC’s budget.

Michael Mulgrew, the head of the teachers’ union, going back on his agreement regarding retiree health-care benefits is further evidence that he cannot be relied upon.

What’s more significant is that he had agreed to transition retirees to Medicare Advantage to create savings that funded salary increases for teachers and exempted United Federation of Teachers retirees from paying health-care premiums. Therefore, if this is no longer an option, Mulgrew must identify alternative sources of savings.

Otherwise, his members may face reduced pay to compensate for the loss.

As per the agreement – approved by the city and the Municipal Labor Committee (representing city unions, including UFT led by Mulgrew) – the 250,000 retirees will transition to a Medicare Advantage plan administered by Aetna, saving $600 million annually.

This deal was negotiated during Mayor Bill de Blasio’s administration; Mayor Adams has been working to implement it.

However, Mulgrew has suddenly backtracked.

“The United Federation of Teachers withdraws its support for the Medicare Advantage program for New York City Medicare-eligible retirees,” Mulgrew wrote in a letter to MLC Chairman Harry Nespoli.

Why the change of heart? Internal union politics, plain and simple.

A group of UFT retirees (many of whom do not reside in the city but still get to vote in union elections) opposed the agreement, leading to a defeat of Mulgrew’s faction in union leadership elections last week.

Mulgrew, who is up for reelection next year, took the hint.

However, employee raises were contingent on the $600 million savings from Medicare Advantage.

The agreement also relieved members from paying health-care premiums.

While the union boss may fear opposition from his members, reneging on the deal and leaving the city with a sudden $600 million annual expense years after the salary increases were implemented is unacceptable.

If Mulgrew chooses to pull out, it is his responsibility, along with the rest of the MLC, to identify new savings to cover the gap.

Would he consider implementing health-care premiums for his members?

Or would he agree to smaller pay increases or even pay cuts for the teachers?

These should be the alternatives if the savings from Medicare disappear and Mulgrew cannot propose a reasonable substitute.

New York taxpayers should not bear the burden of such costs just to protect his position.