The union representing Canada Post carriers is accusing the federal government of violating its members' constitutional rights by pushing emergency legislation to force them back to work.

Labour Minister Patty Hajdu tabled the bill in the House of Commons Thursday morning, triggering a swift rebuke from the Canadian Union of Postal Workers (CUPW).

Canada Post is in its fifth week of rotating strikes by thousands of unionized workers, with no sign yet of a breakthrough in contract negotiations.

Hajdu said the Liberal government is a strong supporter of organized labour and workers' rights and still hopes for a negotiated settlement, but the labour disruption has caused problems for small businesses, people in rural and remote communities and low-income Canadians relying on cheques to pay bills.

Employment Minister Patty Hajdu begins debate in the House of Commons on an act that would enforce the resumption and continuation of postal services. 1:55

"We also have a responsibility to all Canadians and to the businesses that drive our economy, and when the consequences of a work stoppage become so great that they begin to result in serious or lasting harm, we must act," she said.

"When a strike or a lockout is affecting thousands, or even millions of people, the government must intervene."

Holiday pressure

The government claims it has been forced to act because, with Black Friday and the Christmas holiday season approaching, Canadians need to know they can rely on an efficient mail delivery service.

Parcels have been piling up and deliveries from other countries have been suspended, Hadju said.

Prime Minister Justin Trudeau said back-to-work legislation is never the best option and he hopes the government won't need to move forward on it.

"But hearing from businesses, Canadians right across the country, particularly as we approach the holiday season, who are really feeling negative impacts of this ... while we are continuing to hold out hope that there's going to be a settlement or an agreement at the bargaining table, we also have to do what's responsible and prepare for the possible need for legislation," he said during a stop in Calgary.

Mike Palecek, president of CUPW, called it "unconscionable" that a government that professes to champion workers' rights would table this bill.

Constitutional rights breached?

"Obviously we're outraged. It's another violation of our constitutional rights. Free collective bargaining is a Charter right in this country and it's time the governments respect it," he said.

Palacek accused the government of supporting organized labour only when it's convenient and of doing Canada Post's "dirty work." He said the union's legal team will review the legislation.

The Liberals also tabled a motion to pass the bill in a compressed time frame instead of pushing it through the normal three stages of readings and committee hearings, which typically takes several weeks. MPs have begun debate on the motion, which will be followed by debate on the bill itself if a resolution is not reached in the labour dispute.

Canada Post says rotating strikes have created massive backlogs of letters and parcels. (Darryl Dyck/Canadian Press)

NDP MP Daniel Blaikie accused Hajdu of emboldening Canada Post and weakening the union's position.

"How dare she get up in this place and say she's hopeful they're going to reach a deal when she's been threatening for weeks to legislate those workers back to work and taking away their leverage at the bargaining table?" he said.

NDP MP Daniel Blaikie reacts to Employment Minister Patty Hajdu as debate continues around back to work legislation for Canada Post workers. 0:58

Conservative MP John Barlow accused the government of doing "almost something, but not quite anything" by signalling its intent to act without taking immediate action.

On Wednesday, NDP Parliamentary Leader Guy Caron accused the government of bowing to the demands of e-commerce web giants like Amazon and eBay.

In response, Prime Minister Justin Trudeau insisted the government is encouraging both sides to reach a deal, but is prepared to act if there is no "significant progress."

"We have provided conciliation officers, appointed mediators and offered voluntary arbitration. Legislation is not a step that we take lightly," he said.

Past legislation unconstitutional

In 2011, the former Conservative government passed back-to-work legislation for Canada Post workers which was subsequently challenged on constitutional grounds.

According to the website of Paul Cavalluzzo, the lawyer who handled the case, the Ontario Superior Court of Justice ruled in favour of the postal workers five years later, on April 28, 2016, finding the legislation unconstitutional because it violated the workers' freedom of association and expression as guaranteed under the Charter of Rights and Freedoms.

Asked why she believes this back-to-work bill wouldn't violate Charter protections, Hajdu said the Liberals have taken an approach "dramatically different" from the Conservatives' legislation. The former government did not allow for labour disruption and took pre-emptive action that was harmful to the labour movement, she said.

"We have taken every effort over a long period of time to assist these parties to come to a negotiated agreement," she said.

With files from the CBC's Katie Simpson