By Nick Assinder
Political correspondent, BBC News website
Policies on the family and parenting have already become one of the big debates between the political parties.
But Education Secretary Alan Johnson has now opened up divisions on the Labour benches with his suggestion that marriage is not the be all and end all when it comes to raising children.
Mr Johnson is bidding for deputy leadership
The fact that Mr Johnson is addressing the issue is not surprising.
Claims that high levels of youth crime are a result of the breakdown of the traditional family unit and the recent Unicef report putting the UK bottom of a league table for child well-being have served to push the issue to the top of the political agenda.
The parties are falling over each other to come up with policies designed to tackle the issue - David Cameron suggesting fathers should be forced to support their children and the prime minister calling for "intervention" at an early age to tackle problem children.
But there has been an underlying agreement that - while refusing to stigmatise "alternative lifestyles" or single parents - the traditional family structure of a married mother and father raising children is the best option.
Doomed to fail
Mr Johnson has challenged that assumption head on, suggesting other family models can be equally effective and that there is nothing necessarily superior about the traditional unit.
He insists marriage "represents the pinnacle of a strong relationship".
But he also believes: "That does not mean that all children from married couples fare well, nor that every other kind of alternate family structure is
irretrievably doomed to fail".
British children bottom of well-being league
And he is arguing that the debate centred around marriage is looking at the issue through the wrong end of the telescope.
"It's the child that is at the centre of this, it's the parenting that matters, it's not the form of the relationship," he said.
He particularly attacked Mr Cameron's suggestion that tax breaks to encourage marriage were the way forward.
"It's wrong to suggest that tax and legislation makes relationships, it's
not, it's love," he said.
He also branded the old married couples' allowance, abolished by Labour, as "pernicious and judgmental" because it discriminated against the third of all children whose parents are not married.
Mr Johnson, who was brought up by his sister after his mother died when he was 12, is bidding to become deputy Labour leader when John Prescott stands down with Tony Blair.
And his remarks can be seen as part of his campaign to map out a clear agenda of his own on a key policy and which will strike a chord with many voters.
But his views are at odds with those of the Blairite wing of the government which has been increasingly highlighting the benefits of marriage and who believe the Tories may be chiming best with the public mood.
The prime minister, in particular, repeated his "marriage is good" message during his monthly press conference, talking about the best ways of "promoting and supporting" it.
Still, the prime minister appears to have given the all clear for deputy leadership candidates to open up debate on just about any issue they chose.
The danger for the government, however, is that while the frontbenchers and deputy leadership contenders "debate", the Tories will quietly seize the advantage.