Jeremy Corbyn’s stunning victory was born from a cry of protest against neoliberal austerity and the bland centrism of our Party since the New Labour era.

But protest alone won’t win the next election.  As Opposition Leader, Jeremy also has to become Labour’s Proposition Leader with a credible economic alternative to the suffocating neoliberal orthodoxy of cuts, shrinking the state, privatisation and job insecurity.

The starting point must be a strong defence of the economic record of the last Labour government, because otherwise we will be trapped in that neoliberal orthodoxy just as sadly we were in the last Parliament with an austerity-lite policy.  In truth the banking crisis created the mountainous debt and deficit, not ‘Labour over-spending’.

But the alternative to Osborne’s neoliberal mission to shrink the state needs to be much more vigorous than Labour’s limp call since 2010 for “sensible savings” and hints at where Labour cuts might fall. For that left us neither credible to our right on the deficit, nor credible to our left on anti-austerity, losing us the 2015 election.

Equally the alternative is not new Shadow Chancellor John McDonnell’s plan to eliminate the deficit by Labour tax rises rather than Tory cuts.

In my book Back to the Future of Socialism, just published in paperback with a post-election update – citing leading economists and hard-headed evidence – I show that through faster, fairer, greener growth, the public finances can be brought into balance, and the national debt and deficit reduced without further austerity.

That is not some unelectable platform of ‘tax and spend’, but a modernised Keynesianism: the only way to reconstitute the natural majority in Britain for Labour’s values of social justice and a strong economy delivering prosperity for all.

For before the election the Institute for Fiscal Studies noted that if Oxford Economics’ estimate about the amount of spare capacity in the UK economy is correct “the government would not need to implement any additional fiscal tightening after 2014-15 in order to achieve a current budget balance” (i.e. excluding public investment).  Faster growth would cut Britain’s budget deficit and bring down the debt burden.

Yet George Osborne plans to squeeze the economy in the next three years much tighter than he did over the last three. Local government – and through inevitable cuts in its care services, the NHS too – faces catastrophe.

 This has been the slowest recovery from recession in the post war period. Labour should be demanding growth rates of over four per cent per year until the economy is firing on all cylinders again, kick-started by a £30 billion boost to public investment for each of the next two years centred on housebuilding, infrastructure, skills and low carbon investment.

Here starts the credible economic case for abandoning austerity while still tackling the budget deficit. But will Jeremy and John be bold enough to grasp it?

*Former Cabinet minister Peter Hain was Labour MP for Neath 1991 to 2015. Back to the Future of Socialism is published by Policy Press.