There is a new test in town for measuring the social justness quotient of the policy solutions preached most dogmatically by New Labour over the past decade: Have they been continued by a Coalition government that seems with every new announcement to be seeking either to outflank Thatcher or test the credulity of the nation?

In this context, sirens should be wailing in the war-rooms of Labour's leadership candidates after Nick Clegg's speech yesterday signalling the Coalition's easy embrace of New Labour's social mobility agenda. Social mobility has become the more acceptable face of the meritocratic principle, and if Labour continues to cling to the notion that, as both core value and practical tool, it can be equated with fairness, well, perhaps like Alan Milburn they too could feel at home in this Coalition.

"Fairness means social mobility", declared Nick Clegg in his speech yesterday, betraying not for the first time the impoverished definition of fairness that will lie behind the Coalition's claims to be creating a more just society. "Social mobility is a measure of the degree to which the patterns of advantage and disadvantage in one generation are passed on to the next. How far, if you like, the sins of the father are visited on the son." This was presumably meant in a figurative sense but what a miserable choice of phrase. If we must talk about sins, then it is the sins of society that are visited over and over upon our children born into poverty.

The belief that everyone should have equal opportunities to better themselves and to fulfil their talents is so universally accepted nowadays as to hardly bear repeating. Where the political dividing lines have historically existed - though less so recently - is around the diagnosis of the problem and the articulation of the challenge. While Labour espoused the values of equality and solidarity, illuminating the complicity of all in holding some in poverty, the Conservatives emphasised individual over collective responsibility and saw a minimal role for the state as the primary lever of just outcomes.

The attraction of social mobility for politicians is that it side-steps these more fundamental questions around the role of state and society, and relies on loosely-defined concepts such as ability and effort. At the same time it ignores the complex and contentious issues of how talent is defined and rewarded and how the ingredients of effort relate in the first place to social position. It is self-evident that education and health and housing and income all shape in very real ways not only each person's abilities but also their capacity to capitalise on the opportunities that come their way. Yet Clegg is apparently oblivious to his own breathtaking inconsistency, when in these key policy spheres the Coalition is making clear that any concern about just outcomes is eclipsed by their ideological attachment to enforcing market mores and competition.

There seems to be a subtext to the social mobility narrative that it is convenient to perpetuate: the myth of a deserving and undeserving poor. If we can be comfortable in our delusion that everyone has equal opportunities to progress, we can reckon that the poor have only their own fecklessness to blame when they cannot rise above the circumstances of their birth.

But there is an inherent contradiction in the social ladder model. The reality is that the resources and social capital of the better-off mean that the ladder becomes a mechanism for them to bequeath their advantages to the next generation and create a kind of social closure. This dynamic has historically been at the heart of Labour's analysis of society's malaise. It has underpinned the belief that equal life chances require comparable starting points and that the overriding goal must therefore be significant reductions in inequality.

Even if social mobility as means and end could somehow overcome its circular logic, it provides a framework for public policy that is based upon questionable values and ambitions. First, through its unbalancing preoccupation with individual advancement, it subverts essential social goods such as co-operation, mutuality and collective responsibility. Second, as a distributive mechanism it undermines equal worth, defining human value in the narrowest possible way. Third, it promotes a hegemony of middle-class living and values, suppressing rather than celebrating our differences in talents, in preferences, and in spirit. Fourth, it is not concerned with happiness and emotional wellbeing, measuring progress purely in economic terms.

Fifth, and perhaps most important of all, by its own logic, it requires that some fail in order that others can succeed, embedding and legitimising a degree of inequality that harms us all. This is the crucial point for the left, as Labour's belief that everyone should be able to better themselves has historically been expressed through language and policies concerned with the elimination of poverty and rising living standards across the board. This is absolute social mobility. But the social mobility that has held sway in British politics for nearly a decade is relative and symmetrical - upward mobility indicates equivalent downward mobility. It is a dangerous and rather pitiless model of society, and invites a reckless heedlessness to the consequences for us all whenever one person is allowed to fall.

So perhaps Clegg's confusion can become Labour's opportunity, providing the spur for the party to draw a line under its own muddled embrace of social mobility and to rediscover its language of egalitarianism and solidarity, articulated through a political programme that unashamedly prioritises narrowing the gap between rich and poor. Such a shift is long overdue and could be the key to winning back lost support among those who understand that a society where competition, choice and individualism are subordinated to reciprocity, generosity and shared endeavour, is a society where all can prosper.

Rebecca Hickman