Among the highlights in this week's New Statesman (out tomorrow) is David Miliband's most significant political intervention since the Labour leadership contest. In an exclusive essay written in response to a recent piece by Roy Hattersley ("In praise of social democracy"), a strong and vocal supporter of his brother, Miliband denounces a faction that he calls"Reassurance Labour".

Of the brand of social democracy espoused by Hattersley and others, he writes:

For some, this will be seductive. It is what I shall call Reassurance Labour. Reassurance about our purpose, our relevance, our position, even our morals. Reassurance Labour feels good. But feeling good is not the same as doing good - and it gets in the way when it stops us rethinking our ideas to meet the challenges of the time. And now is a time for restless rethinking, not reassurance.

We've heard from the elder Miliband on several issues since his defeat - the crisis of the European centre-left, the rise of the British far-right, the government's NHS reforms - but this is the first time that he's directly addressed the future of the Labour Party.

He attacks "Reassurance Labour's" preference for the central state, warning that "the weaknesses of the "big society" should not blind us to the policy and political dead end of the "Big State". The public won't vote for the prescription that central government is the cure for all ills for the good reason that it isn't."

Miliband also argues that Labour needs to do much more to defend its record in government:

One of the jewels in the crown of Labour's time in office was the rescue of the National Health Service. As the Commonwealth Fund, the London School of Economics and the Nuffield Foundation have all shown, health reforms as well as additional investment were essential to improved outcomes, especially for poorer patients. Where reform was missing, in Scotland and Wales, the improvement was far slower.

Research by the Financial Times shows a similar picture in schooling, where the gap between best and worst schools, which correlates with a wealth gap, was narrowed in the context of rising performance by schools in general.

Our attacks on the Tories will not work if we are not clear about what we did. We should say loud and clear where we made mistakes, but we should also insist that the list of gains far outstripped the mistakes. After all, even David Cameron said on coming to office that Britain was better in 2010 than 1997.

He ends with an unambiguous warning to his party:

The problem with the definition of social democratic politics by the Reassurance Labour tendency is not just that it reduces our chances of election, but rather that its vision is too narrow, its mechanisms too one-dimensional, and its effectiveness too limited. The debate is not whether one side is unprincipled; instead, it is who is right.

To read the full version of David Miliband's essay, pick up a copy of tomorrow's New Statesman, available in all good newsagents.