Colin James, the president of Unifor Local 222, which represents workers at the plant, confirmed on Tuesday morning that his members were back at work.

Union leaders are scheduled to meet with Prime Minister Justin Trudeau in Ottawa on Tuesday afternoon. Trudeau had expressed "deep disappointment" at GM's move on Monday.

The plant closure, which affects 2,973 assembly line workers, would end more than century of vehicle production in the city east of Toronto. It is part of a wider restructuring that will eliminate 15,000 jobs, GM said on Monday.

At a press conference in Oshawa on Monday, Unifor National President Jerry Dias promised "one hell of a fight."

In 2009, GM shut down Oshawa's truck plant, blaming weak demand for pickup trucks and SUVs. The city has a long history of labour unrest. When the truck plant closure was announced, a convoy of some 250 cars surrounded GM's Oshawa operations, temporarily halting production.

During a 1996 strike, amid rumours that the company would try to restart production, a group of workers took over the plant, welding the doors shut behind them.

(Reporting by Allison Lampert in Montreal; Editing by Nick Zieminski)