A majority of MPs in the House of Commons have voted in favour of the Liberal government's motion to fast-track legislation through Parliament that would force Canada Post employees back to work.

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.

The Liberals won the vote 173 to 13 with the help of a handful of Conservative MPs who remained in Ottawa for the vote.

A majority of NDP MPs stood and left the House in protest after their vote, rendering their votes invalid. Some NDP MPs raised a fist in solidarity to postal workers gathered in the public gallery as they left the House.

The NDP had tried to amend the motion, allowing more time for debate, but their amendments failed by a vote of 172 to 38.

MPs will now move into multiple rounds of debates and votes. A final vote on the bill to send the bill to the Senate is expected late tonight, or early Saturday morning. The Senate has passed a motion to sit this weekend. If the Senate passes the bill, it would get royal assent quickly, likely Sunday or Monday, and would come into force at noon ET the next day.

Union leaders have been mounting fierce opposition to the Liberal's legislative move, vowing to fight the government's actions in court and on the streets.

Canadian Labour Congress (CLC) president Hassan Yussuff and Canadian Union of Postal Workers (CUPW) president Mike Palecek held a news conference on Parliament Hill earlier today condemning the federal government's decision and warning that members will mobilize to protest.

Palecek said members are fighting for pay equity and safer working conditions as employees face an injury "crisis."

The Ottawa local of CUPW issued a news release saying members and "allies" were occupying the constituency office of Environment Minister Catherine McKenna to protest the back-to-work bill.

"I think our members are angry. They're seeing their fundamental rights violated," he said. "We have lots of options to protest the actions of this government if they're going to remove our constitutional rights. "It doesn't surprise me in the slightest that our members are taking action."

According to the union, workers and their supporters occupied the offices of seven other MPs including: Finance Minister Bill Morneau, Innovation Minister Navdeep Bains, Minister of Employment Patricia Hajdu, Julie Dzerowicz, Kate Young, Stephen Fuhr and Jonathan Wilkinson.

Palecek said the bill flies in the face of the Liberal government's stated support for organized labour, and union members will fight back.

Yussuff said the government will face a legal challenge if it passes back-to-work legislation.

"Should the government pass the law, we will be back in the courts and we will get a ruling from the courts on whether this government is violating the most fundamental tenet of our constitution: the right of workers to strike in this country," he said.

Labour Minister Patty Hajdu has expressed hope both sides would reach an agreement, saying all Canadians want postal service resumed. She said the negotiations and service disruption have dragged on too long.

"The time has come where action has to be taken," she said.

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

Five years later, the Ontario Superior Court of Justice ruled in favour of the postal workers, 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 the Liberal legislation would not also violate those constitutional rights, Hajdu said Thursday the Liberals have taken a "dramatically different" approach 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.

Hajdu said those strikes are negatively impacting small business, people in rural and remote communities and low-income Canadians relying on cheques to pay their bills.

Palecek, of CUPW, said the government has mischaracterized the disruption to service, as strikes are rotating and essential cheques mailed to seniors and low-income Canadians are still being delivered.

"We haven't shut down the post office, yet now the government is attempting to shut down collective bargaining," he said.

NDP Leader Jagmeet Singh said the work conditions for postal workers have changed dramatically yet the issues of pay inequity and perilous work environment have not been addressed.

"What we would expect is the prime minister to do everything in his power to ensure that there's a free and fair negotiation," he said on Parliament Hill, surrounded by Canada Post employees. "But instead what he's doing is bringing in the worst, most Draconian legislation today, completely undemocratic, and what he's doing is trampling on the rights of working people."

Canada Post had suggested a "cooling off" for negotiations during the peak holiday period, and offered a $1,000 special payment to each employee at the end of that time.

"Canada Post remains at the table to find the common ground needed to reach a fair and reasonable settlement with the union. We continue to operate in an attempt to minimize further service impacts on the many people who depend on us, especially at this time of year," said a statement from the Crown corporation.