The Australian opposition has blocked government plans for a public but non-binding vote on recognising gay marriage, arguing it would better if the issue was decided in parliament.
Prime minister Malcolm Turnbull's conservative coalition government needs the support of the centre-left Labour Party to get enabling legislation through the senate to hold a national vote on gay marriage on February 11.
But a meeting of Labour lawmakers has unanimously decided against supporting the plebiscite, opposition leader Bill Shorten said.
While Labour supports gay marriage, it argues the plebiscite would trigger a divisive public debate. Labour says parliament should decide the issue without asking the public.
"The experts have unequivocally explained to Labour that the plebiscite would cause harm to gay and lesbian people particularly but not exclusively young people," Mr Shorten said.
"Marriage equality, let's make it a reality, let's just get on with it," he added.
The Australian Christian Lobby, which opposes marriage equality, said it was disappointed "that ordinary Australians are being shut out from having a say about the biggest social policy change in a generation@.
The government released draft amendments to the federal marriage law that would be put to parliament if a majority of Australians endorsed gay marriage in the plebiscite. Opinion polls show most Australians support marriage equality.
But gay rights advocates fear that an aggressive scare campaign could result in the plebiscite failing, putting same-sex marriage off the national agenda for decades.
Some conservative lawmakers have said they will vote against gay marriage in parliament even if a majority of Australians support it.
Religious ministers and officials would be allowed to refuse to officiate at same-sex marriages and churches would be allowed to refuse to provide facilities, goods and services for gay weddings. Some government lawmakers argue the same legal protections should be extended to cake makers and wedding singers who object to same-sex marriage.
Attorney-general George Brandis accused Labour of being more interested in scoring a political win over the prime minister than doing the right thing for gay couples.
"Today is the opportunity for the Labour Party to show that it really does believe in marriage equality or whether it's just playing a political game here," he said before Labour made its decision.