The federal government is unlikely to get its same-sex marriage plebiscite enabling laws through parliament after Labor formalised its opposition.
The Labor caucus has confirmed its opposition to the federal government's proposed same-sex marriage plebiscite.
Shadow attorney-general Mark Dreyfus moved a motion in caucus to oppose the plebiscite bill, which was seconded by Terri Butler.
No one spoke against the recommendation and it was carried unanimously.
Labor leader Bill Shorten told the meeting a series of consultations had been held over the past few weeks with mental health experts and other stakeholders.
The overwhelming response had been sincere and straightforward: "Please do not support this divisive, expensive and unnecessary plebiscite, just get on with it and get the parliament to do its job".
Mr Shorten later said mental health experts and families had expressed concern the plebiscite would harm young gay and lesbian people.
"Having met these families, having listened to their stories, I could not in good conscience recommend to the Labor party that we support the plebiscite about marriage equality," he told reporters at Parliament House.
Mr Shorten rejected suggestions it would kill off the issue in the parlimentary term.
"Labor doesn't give up ... there is more than one door to open to achieve marriage equality," he said.
"The easiest way, which this parliament has done for a hundred years, is legislate, debate it."