The Labor Party has voted resoundingly to change its policy to one of supporting gay marriage.

But federal MPs will not be forced to support gay marriage when the issue comes before Parliament next year because the party also voted to endorse a conscience vote for its politicians.

Prime Minister Julia Gillard at the Labor Party's national conference. Credit:Kate Geraghty

The votes were held this morning after passionate debate for and against the changes at the ALP national conference in Sydney.

The Prime Minister, Julia Gillard, had called for a conscience vote and personally moved the amendment backing this.