Tony Abbott has warned that Liberal Party members who cross the floor to support a same-sex marriage bill will be engaging in a "serious attack on the authority" of Malcolm Turnbull.
The former prime minister's sharp comment on 2GB radio came less than a week out from a party room meeting which will discuss further steps to progress the issue.
West Australian senator Dean Smith is working on a marriage equality bill while his lower house colleagues Trevor Evans, Tim Wilson and Warren Entsch have been discussing options with colleagues.
With the bill to enable a plebiscite failing to pass parliament last year, some Liberals want the bill re-committed to parliament, while others say a voluntary postal ballot could achieve the same aim without the need for legislation.
Mr Abbott said the coalition should take its existing policy of a plebiscite to the next election and stand by it now.
"For them to cross the floor to try to ensure the parliament does it, that is real breach of faith with the public," Mr Abbott said .
"It's obviously a dramatic loss of discipline inside the government and it's a serious attack on the authority of the leadership."
He said any change of policy would be a "vote loser".
Mr Turnbull declined to offer his colleagues any advice when asked on Wednesday.
"I talk to my colleagues all the time and we often have discussions about policy issues, and they are best done privately in the party-room," he told reporters in Western Australia.
"I'm not going to get into a public debate about internal party issues and discussions."
The attorney-general's office confirmed in March it had been asked for advice on a postal ballot on same-sex marriage.
Cabinet minister Simon Birmingham, a moderate who has long supported marriage equality, issued a caution about crossing the floor.
"It's always been ... a right in the Liberal Party for individual members to exercise their conscience from the backbench if they choose," he told Sky News.
"But equally, it is, of course, a right that people should exercise carefully, with caution and with consideration for all of the consequences."
Tasmanian senator Eric Abetz - a staunch opponent of same-sex marriage who would vote against it even if a plebiscite showed support - warned it was a "grave matter" for any government to lose a procedural vote in the lower house.
Manager of Government Business Christopher Pyne - who's in charge of running the lower house - dismissed this as "complete rubbish".
MPs pushing for marriage equality argue they have discharged their duty to support a plebiscite and it's not their fault the Senate blocked the legislation.
But Minister Zed Seselja said that argument was nonsense, pointing to the government's persistence over workplace relations laws.