The union representing Canada Post employees is taking the Trudeau government to court over the legislation that ended rotating strikes by its members.

The Canadian Union of Postal Workers said Tuesday it plans to file a constitutional challenge in Ontario Superior Court, arguing Bill C-89 violated the rights of workers to bargain new contracts.

“You cannot legislate labour peace,” CUPW national president Mike Palecek said in a statement announcing the court challenge. “This law violates our right to free collective bargaining under the Charter of Rights and Freedoms.”

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The legal move comes one day after the government appointed a mediator to bring the labour dispute to an end.

Former Canada Industrial Relations Board chair Elizabeth MacPherson has been given up to 14 days to try to reach negotiated contract settlements between the Crown corporation and the union.

The two sides have not been at the bargaining table since the Liberals introduced Bill C-89 to halt rotating walkouts that had sporadically shut down postal sorting plants across the country since mid-October.

The legislation was fast-tracked through the House of Commons and Senate before becoming law Nov. 27.

Failing an agreement between the Crown corporation and CUPW, MacPherson will have the authority to impose a settlement through binding arbitration.

Canada Post said it would “fully participate” in the mediation process while CUPW said it would do so “reluctantly.”

The union’s lawyer said the back-to-work legislation was passed after Canada Post created a “false emergency” over a backlog of parcels at the Crown corporation’s sorting plants.

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“The Liberal government’s legislation, just like the previous Conservative’s, unilaterally prohibits any lawful strike,” Paul Cavalluzzo said in a statement. “This legislation was enacted under circumstances that did not justify the interference of constitutional rights.”

In 2011, the Conservatives under Stephen Harper legislated an end to a two-week lockout at Canada Post. Ontario Superior Court Justice Stephen Firestone later ruled that the legislation violated the rights of postal workers to freedom of association and freedom of expression.

In 2015, the Supreme Court of Canada declared the right to strike to be fundamental and protected by the Constitution.

Both Canada Post and Labour Minister Patty Hajdu’s office said Tuesday they wouldn’t comment on the latest court filing. The government has previously said that while the 2011 bill imposed contract terms on the two sides, this year’s imposes a more evenhanded process.

Canada Post said Monday that, while letter mail is moving well since the rotating walkouts were brought to an end Nov. 27, parcel deliveries are moving slowly and delivery delays are expected through January as a result of the job action and other factors including weather and occasional protests in support of the postal workers.