Prime Minister Justin Trudeau has broken a signature campaign promise by abandoning electoral reform, sparking condemnation from his political opponents.

During the past campaign, Mr. Trudeau made the pledge to "make every vote count" and said his party was committed to ensuring that the 2015 election would be the last under the first-past-the-post voting system. Fair-voting advocates have long argued that the current system doesn't accurately reflect the will of the public, because it allows a party to win a majority of seats without majority support from Canadians. The Liberal promise was part of a series of proposals that Mr. Trudeau vowed would bring real change to Ottawa.

Mr. Trudeau had previously advocated for a ranked-ballot system, but said he was open to a variety of options. The NDP and Green Party pushed for proportional representation, while the Conservatives didn't advocate for change but said any proposed alterations to the voting system had to be put to a referendum.

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The Prime Minister dispatched the newly installed Democratic Institutions Minister Karina Gould on Wednesday to explain why the government is abandoning its promise.

"It has become evident that the broad support needed among Canadians for a change of this magnitude does not exist," Ms. Gould told reporters.

But questions about the government's dedication to the file have persisted for months.

Mr. Trudeau already suggested in an October interview with Le Devoir that Canadians' motivation to change the system had waned since Stephen Harper's Conservatives lost power. Liberals on an all-party parliamentary committee studying the issue refused to sign on to the Dec. 1 majority report, which called for a referendum on an unnamed proportional voting system, calling it "rushed" and "too radical." And Maryam Monsef, who was recently shuffled out of that ministerial role, had to apologize after she belittled the committee's work.

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Then there was the much-maligned MyDemocracy.ca survey, which drew some 360,000 responses, but never asked specifically about voting systems.

Ms. Gould's new mandate letter, released by the government on Wednesday, instructed her that "changing the electoral system will not be in your mandate."

"A clear preference for a new electoral system, let alone a consensus, has not emerged," it says. "Furthermore, without a clear preference or a clear question, a referendum would not be in Canada's interest."

While defending his decision in Question Period on Wednesday, Mr. Trudeau repeated his preference for a preferential, or ranked, ballot system. He pointed to the fact that the NDP wants proportional representation and the Conservatives calling for a referendum as a sign that consensus could not be reached.

"There is no clear path forward. It would be irresponsible for us to do something that harms Canada's stability," he said in a response to a question from NDP Leader Tom Mulcair.

"I'm not going to do something that is wrong for Canadians just to tick off a box on an electoral platform," he added.

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The NDP, which has been pushing for a more proportional voting system, was Mr. Trudeau's harshest critic on Wednesday.

"Justin Trudeau lied to Canadians about democratic reform," Mr. Mulcair said outside the Commons.

Nathan Cullen, the NDP's democratic-reform critic, said he met with Ms. Gould as recently as Tuesday, when she asked him for his thoughts and suggestions on democratic reform.

"What Mr. Trudeau proved himself today was to be a liar, was to be of the most cynical variety of politician, saying whatever it takes to get elected then once elected seeking any excuse, however weak, however absent, to justify that lie to Canadians," he said.

The government abandoned its promise, Mr. Cullen said, when Mr. Trudeau's preferred ranked-ballot system was dismissed by experts. "They're fearful of having the voting system that doesn't keep Liberals in power forever and more," he said.

The Conservatives, who consistently pushed for a referendum, said there was at least consensus on the committee to hold one. Interim Conservative leader Rona Ambrose said, "Canadians should think twice about believing what Justin Trudeau says," she said.

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Green Party Leader Elizabeth May, close to tears, said many people voted strategically for the Liberals based on Mr. Trudeau's promise to change the voting system. "I feel more deeply shocked and betrayed by my government today than on any day of my adult life," she said.

Ms. Gould's mandate letter also asks that she work with Defence Minister Harjit Sajjan and Public Safety Minister Ralph Goodale to defend Canada's voting system from cyberthreats. In addition, she was directed to ask the Communications Security Establishment to analyze risks to Canada's political and electoral activities from hackers and release a public assessment.

Ms. Gould also announced the government's plan to tackle the controversy stemming from the Liberals' cash-for-access fundraisers.

"We believe Canadians have a right to know even more than they do now about political fundraising and we are taking action," she said.

The promised legislation would require cabinet ministers, party leaders and leadership candidates to publicly advertise their fundraisers in advance, and release a report after the fact with details of the event.

Meanwhile, New Democratic ethics critic Alexandre Boulerice tabled his private member's bill that would ban preferential access to ministers and the prime minister at private fundraising events.