Victorian senator Gavin Marshall, a factional convener and chairman of the Left group in caucus, said that while there appeared to be a favourable view of conscience votes in the community, in practice it would not be democratic on this issue.

It ''cedes power to the minority conservative element within the party as they know they can count on the support of an opposition that is stubbornly opposed to all social reforms,'' he writes in The Age today.

It also exposed MPs to powerful conservative lobby groups. A majority would like to vote for equal marriage, but ''a conscience vote has the potential to force many to default to factional alliances rather than vote with their heart''.

Senator Doug Cameron, also a Left convener, said the conscience vote was a political tactic by the PM to ''satisfy some of the more extreme views on the right of the party''.

Ms Gillard said she would not shirk from being involved in the gay marriage debate at the ALP's national conference. She reiterated that there would be no government bill on marriage equality, but said a conscience-vote policy would permit an individual Labor MP to bring forward a private member's bill.

The proposal to give Labor MPs a free vote will prevail at the conference, but it is not clear whether the party platform's wording against gay marriage will stand. Ms Gillard wants the present wording to remain.

Victorian Labor MP Martin Foley said Ms Gillard's position was ''very disappointing and slightly disingenuous''.

''Across the nation we've seen most state branches of the Labor Party resolve that marriage equality is an issue that needs to be dealt with, and now at the last minute we've had the Prime Minister come over the top and say that an issue of fundamental equality is somehow an issue of belief and personal conscience,'' Mr Foley said.

He said it was not an area for a conscience vote. ''For the Prime Minister to argue that this is somehow a special … issue is to fly in the face [of] over 100 years of Labor caucus solidarity.''

Federal Left convener Graham Perrett differed from his factional colleagues. While he strongly supported marriage equality, he said there should be a conscience vote.

Federal minister Mark Arbib, a Right powerbroker who supports gay marriage, backed the conscience vote, saying it meant that in future marriage equality would be achieved. He believed there should be a change in the party platform's wording, but he needed to ''see specific wording and talk to colleagues'' before backing any change.

Paul Howes, right-wing Australian Workers Union chief, said his union's delegates would have a free vote at the ALP conference.

The Australian Christian Lobby said Ms Gillard had tried to hold the line on marriage.

"The PM has clearly tried to deal with the political reality of a section of the party that seems hell-bent on losing forever a huge proportion of the population that cares deeply about marriage, in order to satisfy its deconstructionists' ideology," said managing director Jim Wallace.