David Cameron will rely on Ed Miliband's help to force gay marriage laws through Parliament and in return accept a Labour plan to consider civil partnerships for all.
All the three major parties will support the Government's Marriage (Same-Sex Couples) Bill and reject a "wrecking" amendment backed by up to 150 Tory rebels.
This amendment, proposed by former Conservative minister Tim Loughton, would have delayed the gay marriage laws by bringing in civil partnerships for straight as well as gay couples as part of the Bill.
Both Labour and the Liberal Democrats back the idea of civil partnerships for all. However, they have agreed to vote with Mr Cameron's Conservative leadership to make sure the laws to legalise gay marriage are not hijacked or stalled.