Tony Abbott has issued a stern warning to colleagues pushing for a free vote on same-sex marriage, insisting they are “honour-bound” to oppose it without a plebiscite.

The former prime minister says moves by Liberal backbenchers to draft a bill on marriage equality and push for a parliamentary vote are “fraught”, warning it would dramatically deepen the trust deficit plaguing politicians. He scolded colleagues who “insist on having public second thoughts about our own policy”.

““Keeping faith with the electorate should weigh just as heavily as deeply-held personal belief,” he writes in The Australian.

“Like it or not, coalition MPs are honour-bound to oppose same-sex marriage in the absence of a plebiscite that’s supported it.” Mr Abbott insists the marriage equality backers hold a minority view within the coalition ahead of what’s expected to be a heated special party room meeting on the issue in Canberra on Monday afternoon.

Backbencher Dean Smith has drafted his own marriage equality bill and together with Trent Zimmerman, Tim Wilson, Trevor Evans and Warren Entsch, will push for a parliamentary vote on the bill after circulating it to members over the weekend.

But there’s wide speculation a postal plebiscite proposal will be put to cabinet on Monday and taken to the party room.

Mr Zimmerman says that is possible, but he’s hopeful the rogue MPs can persuade their colleagues.

“I’m always an optimist,” he told ABC radio.

It would be “untenable” for the issue of gay marriage to remain unresolved this year, he said.

Senator Smith’s bill, expected to be supported by Labor, allows for two people to marry regardless of their gender, while enshrining protections and exemptions for religious communities.

That includes religious ministers, civil celebrants, and businesses linked to a religious body who refuse to marry same-sex couples.

Senator Smith believes it’s the most comprehensive marriage equality bill to go before parliament.

And it’s been given the green light by advocates, who believe it’s one of the strongest they’ve seen.

“It is a bill that is designed to pass the Senate,” Australian Marriage Equality co-chair Alex Greenwich said.

It’s expected many MPs will continue their push for a plebiscite, which the coalition promised at the last election but has failed to get it through parliament.

Others are tipped to argue for a non-binding postal vote being touted by cabinet minister Peter Dutton.