Nationals MP Andrew Broad has signalled he would withdraw support for the government if there is an attempt to legislate marriage equality without a national plebiscite.
"Let me be very clear, the government that I am a part of and will remain a part of, is conditional that the only way that there will be a change to the Marriage Act in this parliament is a plebiscite," he told reporters on Tuesday morning.
"My position as a member of the government is very clear on that. That if there is going to be changes to the Marriage Act, it’ll be a plebiscite in February next year."
The Turnbull government currently has a one-seat majority and would be thrown into turmoil if Broad or any other lower house MP chose to withdraw their support.
His comments are a warning to high-level government moderates, including prime minister Malcolm Turnbull and attorney general George Brandis, not to ram through marriage equality in the parliament any other way.
Today, Labor is expected to announce it will block the plebiscite in the senate.
LGBTI advocates, who largely oppose a plebiscite, will continue to push for a free vote in this term of parliament.