The powerful New York teacher’s union has escalated its latest beef with Mayor Bill de Blasio, urging parents to demand school closures over coronavirus concerns.

“Because of his irresponsible decision to keep the public schools open, Mayor Bill de Blasio can no longer assure the health and safety of our students and school communities,” wrote Michael Mulgrew, president of the United Federation of Teachers, in an email to its members.

“The mayor is recklessly putting the health of our students, their families and school staff in jeopardy by refusing to close public schools. We have a small window of time to contain the coronavirus before it penetrates into our communities and overwhelms our health care system’s capacity to safely care for all the New Yorkers who may become gravely ill,” he continued.

The UFT wants parents to call 311 — which has already been flooded with calls regarding the coronavirus — to demand that the city close schools and maintain services for “medically fragile students and other vulnerable children.”

Teachers have already called for a “mass sick out,” urging teachers to call in sick to defy the mayor — who has repeatedly defended his stance that schools should remain open.

More than 1.1 million students attend more than 1,900 public schools in the city.