OTTAWA (Reuters) - Canadian opposition parties accused the government on Thursday of reneging on a promise to make the country's voting system fairer after a key Liberal minister dismissed an official report that recommended having a referendum before changes are made.

The denunciation of the report that the Liberals had asked for could kill the momentum for an overhaul that was expected to benefit smaller parties like the left-leaning Greens, who have just one seat in parliament.

Prime Minister Justin Trudeau promised during the 2015 election campaign that Canada would have a new voting system in place by the next election in 2019 - suggesting that proportional or preferential voting could come to Canadian politics.

Critics say Trudeau is less enthusiastic about reform now that he has won a majority under the current first-past-the-post system.

An all-party committee recommended on Thursday that Canada should hold a referendum before making changes, something the Liberals have said is not necessary.

The referendum would allow Canadians to choose between the current system and a type of proportional voting, although the report left it up to the government to choose which type.

Minister of Democratic Institutions Maryam Monsef, who has been tasked with electoral reform, criticized the report and the work of the committee for a lack of consensus and not recommending a specific alternative.

The opposition accused the Liberals of self-sabotage.

"Minister Monsef and Justin Trudeau are trying to find a way out of this because they don't like the answer they got," said interim Conservative leader Rona Ambrose.

The existing system, inherited from Britain, allows a party to win a majority government with less than 40 percent of the popular vote. The Liberals received 39.5 percent of the vote in 2015 but 184 of the 338 seats in the House of Commons.

Proportional representation, used in countries like New Zealand, would likely boost the performance of small parties by allotting seats based on the popular vote rather than by a candidate's performance in each electoral district, as in the first-past-the-post system.

Trudeau already appeared to be backing away from his promise to reform the system, telling Le Devoir newspaper in October that the timetable was likely to slip. He also said major changes would need the substantial backing of Canadians.

"The Liberals have been doing a remarkable job of hedging and backpedaling," said David Moscrop, electoral reform researcher. "They might just let it die on the vine."

(Reporting by Leah Schnurr; Editing by Peter Cooney)