Parliament is heading for a historic vote on same-sex marriage in which all MPs will be free to vote according to their conscience.
The Coalition party rooms are likely to decide on a conscience vote during the coming spring session of Parliament, with one Liberal MP saying it is now ''almost certain'' the party will dump its binding opposition to gay marriage.
Prime Minister Tony Abbott, whose sister Christine Forster is in a same-sex relationship, promised before the election the Liberal Party room would be free to decide on a conscience vote.
The matter is expected to reach a crucial stage in the next two sitting fortnights of Parliament in August and September after senior Liberals asked crossbench Senator David Leyonhjelm to introduce his draft bill to legalise same-sex marriage.
The government wants his proposed legislation on the notice paper so that it has time to scrutinise the exact wording before deciding which way to go.
Senator Leyonhjelm has agreed, reversing his stated position when he announced the draft bill last month. At the time, he said he would not introduce it until the Liberal Party room opted for a conscience vote.
Senator Leyonhjelm said: ''I have heard from Liberal senators that a conscience vote is highly likely.''
But he said he wasn't ''counting my chickens'' on a vote for same-sex marriage on the floor of Parliament and a number of backbenchers said their ''gut feeling'' was that it would be narrowly defeated even with a conscience vote.
Opposition Leader Bill Shorten is a strong advocate of same-sex marriage and Labor MPs have been granted a conscience vote. But a proposal that would call on them to bind in favour of such marriages was defeated at last week's NSW Labor conference.
A Liberal MP, who would vote against same-sex marriage, said: ''I don't think even Tony Abbott will stand up in the party room and argue against a conscience vote. It would appear to go against Liberal principles. I don't think you would find any of my colleagues who would say a conscience vote was a bad idea.''
Another MP said the party would prefer to deal with Senator Leyonhjelm's bill than delay and potentially deal with ''a more radical proposal put up by the Greens''.
Senator Leyonhjelm, who represents the Liberal-Democrats, has appealed to the libertarians in Liberal ranks to back same-sex marriage. ''If it doesn't get through I have six years to poke them,'' he said. ''At some point they will see I am not going to give up on this.''
National Party MPs are expected to vote against same-sex marriage. Christian groups are understood to have a war chest ready to fight against any move to legalise such marriages.