Joan Jessome, president of the Nova Scotia Government and General Employees Union, said the nurses will go on strike if Stephen McNeil's Liberal government brings in such a law.
"If they bring in legislation, there will be a strike," Jessome said, adding that the nurses are prepared to hit the picket lines before they are in a legal strike position April 3.
"If legislation were to come in tomorrow, there would be a strike tomorrow."
Mediated talks between the union, which represents about 2,400 registered nurses, and the Capital District Health Authority reached an impasse Sunday evening.
McNeil wasn't available for comment Monday, but he has said back-to-work legislation is a possibility in the nurses' labour dispute.
Earlier this month, 420 striking home-care workers were ordered back to work after the province passed essential service legislation.
Jessome said she hopes a negotiated solution can be reached by April 3, but adds that she is losing confidence that the government will compromise on the union's request to hire more nurses.
The union has said it wants nurse-to-patient ratios, something it says would improve patient safety. But Capital Health says there is no evidence that shows mandated registered nurse-to-patient ratios guarantee improved patient safety.
Kathy MacNeil, vice-president for people services at Capital Health, said the health authority is worried about the possibility of an illegal strike occurring before the hospitals have an emergency plan in place.
"If something happened unexpectedly like an illegal work stoppage, it would have some serious impacts on our ability to receive patients from other parts of the province and other parts of the Maritimes," MacNeil said.
She said the threat of a walkout means the health authority will have to speed up its plans to cancel surgeries and return patients to hospitals outside of Halifax.
Dr. David Kirkpatrick, chief of surgery at Capital Health, said if a strike happens, hospitals will continue with approximately 20 emergency surgeries daily, but will cancel some other elective procedures.
Kirkpatrick said organ transplants could be cancelled due to the lack of a clinical co-ordinator, who arranges the matches and transportation of the organs.
"We haven't had a clear message from the union they will provide that person during the strike," he said.
Jessome said organ transplants will continue in the event of a strike. The union also issued a news release saying nurses will continue to staff basic coverage areas including the emergency room, intensive care units, veterans' care, hemodialysis and cancer care.
The union said there were about 40,000 "patient safety events" reported in Capital Health between April 1, 2011, and March 31, 2013. Jessome said there are links between those incidents, which ranged from medication errors to patient deaths, and lower staffing levels.
MacNeil said more than half of those incidents caused no harm to patients.
"We do have 40,000 incidents," she said. "The majority are near-misses. It could be things like mislabelling a specimen or an armband or a near slip or fall."
She said studies have not shown a clear link between staffing levels and the incidents.
The nurses primarily work at four places in the Halifax area: the QEII Health Sciences Centre, Nova Scotia Hospital, East Coast Forensic Hospital and Public Health Services.