Australian opposition MPs have said they will block the prime minister's plan to hold a national vote on legalising same-sex marriage.
Malcolm Turnbull has promised to put the issue to a non-binding ballot, or plebiscite, next year.
But critics, including many supporters of same-sex marriage, say parliament should make the decision itself.
They say the plebiscite will be expensive and runs the risk of unleashing homophobic rhetoric.
Parliament will still vote on whether to hold the plebiscite, but without opposition and crossbench support it is unlikely to pass the senate.
Mr Turnbull has not yet said whether he will allow MPs a free parliamentary vote instead, as the opposition wants, meaning the issue is unlikely to be resolved soon.
What are the basics of the plan?
Australia's Marriage Act currently specifies marriage as a union between a man and a woman. If parliament approves a vote, Australians will be asked next February: "Should the law be changed to allow same-sex couples to marry?"
How many people will be affected?
According to the 2011 census, there were around 33,700 same-sex couples in Australia - about 1% of all couples in Australia. Opinion polls show between 60% and 72% of Australians support legalising gay marriage
Is there a different way the issue can be decided?
The opposition say campaigning around the plebiscite could allow aggressively homophobic language. A no vote could also set the legal changes back for years to come.
They pushing for the matter to be settled through a free vote in parliament, where politicians would vote according to their individual beliefs.
Read more: Australia gay marriage in the spotlight
The vote is expected to cost A$160m (£95m; $120m) which includes equal funding for both the yes and no campaigns.
Opposition Labor leader Bill Shorten said on Tuesday that his party would oppose the "expensive, divisive plebiscite".
"Why should a couple in a committed relationship have to knock on the doors of 15 million of their fellow Australians and see if they agree with it?" he said.
"The easiest way is the way which this parliament has done for a hundred years - legislate."
Mr Turnbull is a long-time personal supporter of same-sex marriage but has said the public should have a say.
He said on Tuesday that the opposition was "not so much interested in same-sex couples being able to marry as they are in wringing every ounce of political gain out of this debate".