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.

"They will see I am not going to give up on this": Senator David Leyonhjelm. Credit:Alex Ellinghausen

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.