The Canada Industrial Relations Board (CIRB), a federal watchdog, ruled on Wednesday that the Pacific coast dock workers' strike must end because their union did not provide the required 72-hour notice before the walkout.
"This strike is illegal," Labour Minister Seamus O'Regan said on Twitter after the ruling. The International Longshore and Warehouse Union (ILWU) had no immediate comment.
On Tuesday the ILWU leadership rejected a tentative four-year contract deal agreed with employers last week that ended a 13-day strike, and said the 7,500 dock workers would return to the picket line.
"My patience has run out," Transport Minister Omar Alghabra told reporters in Argentia, Newfoundland, on Wednesday when asked if the government would pass back-to-work legislation, a politically tricky move.
The government is exploring all its options to end "disruptions to our largest export and import gateway to the world," Alghabra said.
The ruling may eliminate the need for back-to-work legislation at least for a few days.
The strike has upended operations at two of Canada's three busiest ports, the Port of Vancouver and the Port of Prince Rupert, which are key gateways for exporting natural resources and commodities and bringing in raw materials.
The walkout is estimated to have disrupted C$6.5 billion ($4.9 billion) of cargo movement at the ports, based on the industry body Canadian Manufacturers & Exporters' calculation of about C$500 million in disrupted trade each day.
The leader of Canada's Conservative Party, Pierre Poilievre, earlier called on Liberal Prime Minister Justin Trudeau to come up with a plan to end the strike within 24 hours, but did not say whether he would support back-to-work legislation.
The left-leaning New Democratic Party (NDP) has been helping Trudeau's minority government pass legislation in parliament, but on Wednesday NDP leader Jagmeet Singh ruled out support for legislation forcing workers back.
"Don't signal in any way that you're going to bring a back-to-work legislation because that's going to undermine the workers," Singh told reporters was his message to the government when asked about back-to-work legislation.
That means Trudeau would need the votes of the Conservatives, who have been trying to court workers and unions in recent months, to pass back-to-work legislation, or the Bloc Quebecois, whose leader has yet to comment.
It also means the Liberal-NDP deal that is keeping the government going could be put under strain if Trudeau chooses to force an end to the strike
($1 = 1.3181 Canadian dollars)
(Reuters - Reporting by Ismail Shakil in Ottawa; Editing by Chizu Nomiyama, Andrea Ricci and Chris Reese)