Just after 2 p.m. on Monday afternoon, Unifor officially confirmed the walkout. CBC Thunder Bay reporter Jeff Walters said employees were starting to leave the plant.
Earlier on Monday, the union representing about 1,000 employees at the plant said the company rejected its latest offer, and had not come back to the bargaining table.
"We're waiting to see if the company will come back to us with an amended proposal at the final hour," said Unifor national representative Andy Savela.
"We hope to avert a strike, but, at this point, we haven't heard much today."
Savela said if no last minute deal was reached, workers would be on the picket line at 2 p.m.
A spokesperson for Bombardier told CBC News the company will "be in touch" later on Monday regarding the labour situation.