A SENIOR Liberal has urged that his party's MPs should be given a conscience vote when gay marriage legislation comes before Parliament next year.

Shadow parliamentary secretary Simon Birmingham said this would be in the best interests of the Liberal Party ''on a matter where so many views are informed by moral or ethical considerations''.

Senator Birmingham, from South Australia, said he would vote for gay marriage if there was a conscience vote for Liberals, subject to seeing the details of the legislation. He would not be drawn on how he would vote if the party did not allow a conscience vote.  With the Labor conference having supported gay marriage on Saturday, while deciding MPs would have a conscience vote, the party yesterday stepped up pressure on Tony Abbott to allow his MPs the free vote.

PHOTO: A SENIOR Liberal has urged that his party’s MPs should be given a conscience vote when gay marriage legislation comes before Parliament next year. Shadow parliamentary secretary Simon Birmingham said this would be in the best interests of the Liberal Party ”on a matter where so many views are informed by moral or ethical considerations”.

Defence Minister Stephen Smith said Mr Abbott would find it hard to say to his parliamentary party: ''Here is the decision of the shadow cabinet - you are now all bound by that.''

Mr Smith said that if the Coalition gave a conscience vote, there was a chance that a private member's bill on gay marriage could get through Parliament.

Human Services Minister Tanya Plibersek said: ''I'm very much looking forward to seeing Tony Abbott give his MPs a conscience vote … I think there are a lot of people in the Liberal Party who also believe in marriage equality.''

The Labor Party and the Greens are vying to sponsor a private member's bill on gay marriage. The Labor Left does not want the Greens to get credit for carriage of the issue in Parliament.

A bill is doomed if the opposition votes as a block. Although the opposition is considered certain to collectively oppose the legislation, rather than having a conscience vote, Mr Abbott refused to be pinned down.

''There's a sense in which every vote in the Liberal Party is a conscience vote because we don't expel people for exercising their judgment, unlike the Labor Party,'' he said. He added that if and when legislation came before Parliament, it would go through ''our standard party room process and at that time everyone will have a full opportunity to say their piece''.

Previously, Victorian moderate Russell Broadbent has said he favours a conscience vote.