Gay marriage opposed by Welsh ministers David Jones and Stephen Crabb
Welsh Secretary David Jones will vote against plans to legalise gay marriage in Wales and England.
His fellow Conservative and Wales Office deputy Stephen Crabb will also oppose the move.
Prime Minister David Cameron faces criticism from some Tories for supporting the idea, with Monmouth MP David Davies calling it "barking mad".
Mr Davies said the party would lose loyal activists if it pressed ahead with the policy.
BBC Wales political editor Betsan Powys reports from the Senedd.