PROVIDENCE — Butler Hospital and the union representing nurses and other workers at the East Side psychiatric facility reached a tentative contract agreement late Wednesday, averting a threatened strike.

Members of the union representing nearly 300 nurses, mental-health workers and other employees had voted earlier in the week to strike starting yesterday morning if their contract demands were not met. The members of District 1199, the New England Health Care Employees Union/SEIU picketed the hospital earlier in the week to call attention to the contract issues that remained unresolved at that point. The nurses had been working under an extension of a three-year contract since March 31.

But just before midnight Wednesday, they reached an agreement with management that will keep them on the job.

A key sticking point was the hospital’s use of “travel nurses,” who work for staffing agencies. The union accused hospital management of using the temporary nurses to fill vacancies.

Hospital management says the use of travel nurses is rare but necessary.

Hospitals have used travel nurses since the early 1980s, when a nursing labor shortage and nurse specialization spawned the specialty. Initially, nurses were brought in to staff hospitals to fill both permanent and seasonal shortages. As travel nurses became more embedded in the nursing industry, their work expanded to fill on-demand positions in specialized nursing areas.

The 117-bed Butler Hospital has used travel nurses in the past, according to a hospital spokeswoman. About 2 percent of the nursing shifts are filled by the temporary workers.

Care New England, the organization that operates Butler Hospital, uses travel nurses in at least one of its other facilities — Kent Hospital in Warwick. Care New England also includes Women & Infants Hospital and Care New England Home Health.

Neither the union nor the hospital yesterday would release details of the contract proposal, though a union representative sounded encouraged about the tentative, three-year, pact.

“It addresses many of our concerns,” said Stan Israel, executive vice president of the New England Health Care Employees Union.

Union members are expected to review the proposal today at the hospital in Providence, beginning at 7:30 a.m. Voting will take place throughout the day, ending at 4:30 p.m., Israel said.

pgrimald@projo.com