The walkout involves as many as 3,500 nurses at Montefiore Medical Center in the Bronx and around 3,600 at Mount Sinai Hospital in Manhattan.

The New York State Nurses Association, which represents the workers, said it was being forced into the drastic step because of chronic understaffing that leaves them caring for too many patients.

“Nurses don’t want to strike. Bosses have pushed us to strike by refusing to seriously consider our proposals to address the desperate crisis of unsafe staffing that harms our patients,” the union said in a statement late Sunday.

The hospitals have been getting ready for a walkout by transferring patients, diverting ambulances to other institutions, postponing nonemergency medical procedures and arranging to bring in temporary staffing.

Gov. Kathy Hochul urged the union and the hospitals late Sunday to take their dispute to binding arbitration.

Montefiore’s administration said in a statement that it was willing to let an arbitrator settle the contract “as a means to reaching an equitable outcome.”

The union didn’t immediately accept the proposal. In a statement, it said Hochul, a Democrat, “should listen to the frontline COVID nurse heroes and respect our federally-protected labor and collective bargaining rights.”

Montefiore and Mount Sinai are the last of a group of hospitals with contracts with the union that expired simultaneously. The Nurses Association had initially warned that it would strike at all of them at the same time — a potential calamity even in a city with as many hospitals as New York.

But one-by-one, the other hospitals struck agreements with the union as the deadline approached.

Nurses at NewYork-Presbyterian Hospital ratified a deal Saturday that will give them raises of 7%, 6%, and 5% over the next three years while also increasing staffing levels. That deal, which covers 4,000 nurses, has been seen as a template for the negotiations with other hospital systems.

Nurses at two facilities in the Mount Sinai system also tentatively agreed to contracts Sunday. But negotiations continued at the system’s flagship hospital on Manhattan’s east side.

Mount Sinai’s administration said in a statement that the union’s focus on staffing-to-patient ratios “ignores the progress we have made to attract and hire more new nurses, despite a global shortage of healthcare workers that is impacting hospitals across the country.”

If the nurses strike, patients are likely to see disruptions in care such as emergency room visits and childbirth.