Updated at 3:28 p.m. April 27

The nurses’ union at St. Vincent Hospital in Worcester has filed an unfair labor practice charge with the National Labor Relations Board, alleging the hospital violated the new labor agreement that ended the union’s historic 301-day strike.

The Massachusetts Nurses Association argues it has a legal right to negotiate shift changes for its members. However, the hospital recently notified nurses in March that many of them must begin working 12-hour shifts starting May 1. The union is seeking an injunction from the National Labor Relations Board to prevent the plan from taking effect.

Although 12-hour shifts are not unusual at other hospitals, union officials at St. Vincent note the new labor agreement stipulated that nurses would return from the strike to their former positions and shifts. If the hospital wants to change shifts now that nurses have returned, union officials say management has to negotiate the changes with the union.

“You are basically strong-arming professional registered nurses into working shifts — longer shifts they never had — which just may not be feasible for [some] nurses,” said Marlena Pellegrino, co-chair of the nurses’ union bargaining committee.

In a statement, St. Vincent Hospital said current nurses and applicants for new positions have told hospital management they prefer working three 12-hour shifts per week instead of five 8-hour shifts.

“Our decision to transition to twelve-hour shifts is to enable the hospital to attract and retain as many nurses as possible,” the hospital said in the statement, adding the labor agreement does allow St. Vincent to make the change without negotiating with the nurses' union.

David Schildmeier, a spokesperson for the union, acknowledged there's a provision in the labor contract giving the hospital the right to move nurses to 12-hour shifts, but argued another clause in the agreement ensures the union can negotiate over the change. He said the hospital held a meeting in April with union representatives to discuss the shift changes, but negotiations were unsuccessful.

“They heard our proposal and mentioned they would participate in a follow up session, and subsequently reneged on that commitment, which prompted our filing for the injunction,” Schildmeier wrote in an email to GBH News.

The National Labor Relations Board on Wednesday afternoon said they are currently investigating the charge.

The nurses' union said the hospital's plan could backfire and actually cause an exodus of nurses' from St. Vincent. The shift change could impact 250 of its members in inpatient units and the emergency department, union officials said. Pellegrino who currently works from 3 to 11 p.m., is one of those nurses. She cares for her elderly mother in the morning and early afternoon before her shift at the hospital. If the new shifts take effect as planned, she said she’ll either have to work all day, starting at 7 a.m., or all night, beginning at 7 p.m.

“I will be considering employment elsewhere,” Pellegrino said, noting that many other nurses are telling her they plan on doing the same.

The filing is the latest disagreement between the nurses’ union and St. Vincent Hospital. Last year, the union went on strike for nearly 10 months until securing a new labor agreement with the hospital. Then in February, St. Vincent’s CEO Carolyn Jackson expressed support for a campaign to decertify the nurses’ union, which ended up failing.

Pellegrino said the nurses’ union is still willing to negotiate with the hospital over the shift changes. She said although many nurses don’t want the longer shifts, others do.

“We need to have a mix of 8- and 12-[hour shifts],” she said. “Some nurses need 8-hour shifts for a certain reason, and why alienate those nurses?”

This story was updated with a statement from St. Vincent Hospital and confirmation that the NLRB is investigating.