HALIFAX -- The union at the Halifax shipyard where the "backbone" of the next generation of Royal Canadian Navy vessels are being built has given 48-hour strike notice, with picket lines expected to go up Saturday morning.

Unifor issued a news release Thursday saying a strong majority of 850 unionized employees at Irving's Halifax Shipyard have rejected a tentative contract.

Marine Workers Federation Local 1 said in the release that 75 per cent of its members voted against the deal offered following eight months of negotiations.

The local said the four-year tentative agreement that was rejected included increases of 1.5 per cent per year over the next four years.

Unifor said the employer did not agreed to paid sick days for workers, however.

"This is not just about economics, it's about respect for workers and fixing the workplace for members and they are clearly sending a strong message to Irving today," Jerry Dias, Unifor's national president, said in the statement.

A spokesperson for J.D. Irving was contacted by email and telephone, but the company said Thursday morning it was still working on a public response regarding the looming labour dispute.

Larry Haiven, a professor emeritus of labour relations at the Sobeys school of business at Saint Mary's University in Halifax, said in an interview that it appears the tensions have been brewing for some time.

"This comes after several years of management tightening the screws. There's a lot of baggage that comes into this set of negotiations," he said.

"It's not just the pay levels or the benefits. There's a lot of bad blood, a lot of disciplinary actions. There's a lot on the table beyond the usual."

Still, the 69-year-old observer of Halifax's labour scene said it's difficult to predict what will unfold in the closing hours before the anticipated walkout.

"You never know until the 11th hour and even past it," he said.

Haiven said while the union membership may be "angry," the right offer could still prevent a strike.

One factor that may encourage a deal is that the shipbuilding yard is on a tight schedule to produce the order for the federal government, said Haiven.

"The fact there are orders on the books and nobody wants to see the place shut down for awhile is going to push both sides," he said.

According to a news release issued on May 30 by J.D. Irving, Canada's first Arctic and offshore patrol ship is set to launch this fall.

The shipyard said it is planning to build up to six Arctic and offshore Patrol Ships and 15 Canadian Surface Combatants over the next 25 years, referring to the projects as "the backbone of the Royal Canadian Navy."

The first three ships, Harry DeWolf, Margaret Brooke, and Max Bernays, are currently under construction at the shipyard.