Today's report by the Sutton Trust provides further hard evidence of this catastrophic policy failure for a party of the centre-left.
Of course it wasn't Labour that started it. The decline in social mobility and emergence of a British underclass over the past 30 years is first and foremost the legacy of Margaret Thatcher. But the fact that the gap has continued to widen in the past ten years is proof, if ever it were needed, that the role of New Labour has essentially been to perpetuate the Thatcherite settlement rather than challenge or overturn it.
Some people will point to the demise of the Grammar Schools as a factor in preventing children moving out of deprived backgrounds. Others will blame house prices. Others will fatalistically conclude that the establishment always reasserts itself, and that the effortless superiority learned at public school will always be worth more in the job market than countless A-grades.
Either way, the political upside is that there is a challenge here for Gordon Brown which, if he can grasp it, might even yet give his government the moral purpose it currently lacks, and a way back from the political malaise in which it finds itself.
There is also, if his pride will permit, an old adversary who could help in that task - former Cabinet minister Alan Milburn, who was warning about this as long ago as 2003.
Back then Milburn wrote: "Getting Britain socially moving demands a new front in the battle for equal life chances. The most substantial inequalities are not simply between income groups but between those who own shares, pensions and housing and those who rely solely on wages or benefits."
It was designed as a possible prosepctus for the third term. Four years on, is it too much to be hoped that such ideas could yet form the basis of Labour's programe for a fourth term in power?