THE Labor Left will take on the Prime Minister, Julia Gillard, over gay marriage by pushing the party to adopt a policy of outright support rather than a conscience vote.

Firing the first salvo ahead of the ALP national conference in three weeks, the federal minister and senior figure in the Left, Mark Butler, said a conscience vote should not apply to same-sex marriage because it is not a ''life-and-death'' issue like abortion, euthanasia or stem cell research.

Labor policy opposes same-sex marriage and supports the Marriage Act definition as a union between a man and a woman.

Ms Gillard, who personally opposes same-sex marriage, is expected to make a statement before the national conference supporting changing the policy to that of a conscience vote.

The Right, including the ultra-conservative sub-faction of the Right allied to the Shop Distributive and Allied Employees Association, is lining up to support a conscience vote.