Brexit creates significant new challenges for the British left. It torpedoes some great gains, making discretionary various social, political, economic and environmental rights that have become mandatory over the past two generations. And it creates a whole new series of traps for progressive politics.

But Brexit also reflects the weakness of the left in the UK. In fact, Brexit was only possible because of Labour’s shift over the past ten years from a powerful governing majority to a secondary influence on national decision-making. The future of the left is about more than Labour, and about more than parliamentary representation, but only if we understand this shift can the challenges of Brexit be addressed.

Ten years ago Labour in Britain defined the contours of political debate. We had won three elections on the trot and the Tories felt the need to dance to our tunes – from the minimum wage to tripling of overseas aid to gay rights to boosting the National Health Service. Now Labour sits a long way from power, even before boundary changes. The ultimate ignominy of not being able to organise our own party conference has been avoided, but we have not been further from power since the 1930s.

The shift from the mainstream to the margins has not been the product of a series of unfortunate accidents. That would be a reason for sorrow. Yet frustration or anger is more appropriate, because the political situation of Labour is the product of a series of choices. Some of them have been small, others large, but together they have turned the party inwards rather than outwards, looking to the past rather than to new ideas, resting on easy rhetoric rather than taking hard decisions – and above all seeking to distance ourselves from our time in government, rather than building on it, in terms of both policy content and political culture and dynamic.

The party has ended up pre-New Labour in policy and culture, when we need to be post-New Labour. This year’s leadership election has spent a lot of time debating how to “bring back” various lost icons, such as nationalised railways, rather than focusing on new ideas for the future.

The main charge against Jeremy Corbyn is not just that his strategy is undesirable because it makes the party unelectable. That is only half the story. The real issue is that his strategy makes the party unelectable ­because it is in many aspects undesirable.

This is true most egregiously with regard to foreign policy. The half-hearted message about Europe is a betrayal of millions of working people. The equivocation on Nato in the face of Russia’s intimidation of nations

in her former sphere of influence is dangerous and throws away progressive values.

But the electorate can see through the domestic policy, too. Nationalisation cannot be the answer to everything; anti-austerity speeches cannot explain everything; cor­porate taxation cannot pay for everything. It doesn’t add up. It wouldn’t work. People are not stupid.

There is one other element that is not only undesirable, but disastrous. It is the critique that everyone who disagrees with Jeremy Corbyn is in fact a closet Tory – or “Tory lite”. The US Republicans have a similar problem, with anyone to the left of the hard right called “Rino”, meaning “Republican In Name Only”.

The “Tory lite” allegation starts with a fact: government involves compromise. It then fashions an explanation: that the compromise is based on bad motives. It then develops a theory: that the trajectory of our country has been unchanged by Labour government since the Thatcher years. It then creates a new version of history: there is no difference between Labour and Tory governments. This is the sectarianism that leads to the dead end of permanent opposition.

The truth is that global markets, or ­“turbocapitalism”, are rewriting the rules in the economy and beyond. The extremes of income, wealth, innovation, degradation, inspiration, cruelty and humanity of this, the first period in history of truly global capitalism, are reflected in our politics. Globalisation has created inequalities of income, wealth and power that challenge the traditional answers of the centre left. And globalisation has challenged social norms in a way that has broken the back of the traditional centre right.

So the left needs to renew itself in the same way as during three previous periods in the wilderness – first in the 1930s, then in the 1950s, then in the 1980s/1990s. Each time three questions were defining. Only when the left found the right answer to all three did it become a party of government.

The first question is whether the left puts values above doctrine. In other words, is it dogmatic about ends (values) or means (doctrine)? When Labour becomes trapped by policy positions, it loses, because the public thinks it puts dogma above ideals. When it puts values in the driving seat –what is sometimes called “ethical socialism” – then policy imagination is the result. When it is willing to use markets and the voluntary sector as well as the state as agents of change, the left in Britain and around Europe has shown the capacity not only to win the confidence of the public but also to change the country. This is especially important when politics is in such flux. Theresa May’s nods towards a more equal society show the power of our values. But the public wants new and effective ideas to achieve them.

The second question is whether the left has policies for wealth creation as well as fair distribution. This is especially important today because there is so much evidence that inequality is economically inefficient. But tackling inequality is necessary, yet not sufficient to grow the economy. The ­former US treasury secretary Larry Summers’s work on “secular stagnation” shows the demand-side and supply-side dangers facing the global economy. Policy development is vitally needed in both areas to help avoid the trap of low growth and high inequality. More public spending on its own does not spring this trap – and, in the absence of serious ideas for raising productivity, will not work. A small example: “city regions” were a Labour idea ten years ago to drive economic growth from the ground up, long before George Osborne discovered the “Northern Powerhouse”, but were not developed.

The third is whether the left has an international perspective as well as a national one. In the 1930s this was about appeasement. In the 1980s it was about Europe. And today Europe is again the fulcrum. The temptation will be to chase Euroscepticism. This would be a huge error. Now is the time to set clear tests for the government’s negotiations with the European Union, to show how a progressive approach to engagement with the EU helps manage globalisation, rather than turn our backs on it. Nationalist isolationism of the left (or the right) offers no answers in an age of interdependence.

The issues today are momentous: whether global capitalism has more bust than boom; whether we can sustain Western liberal values in the face of global pressures; whether the climate crisis is past resolution; whether the people I work for – refugees and the displaced – will ever find a home; whether public services can survive the flight of capital from Western tax authorities; whether democratic norms can survive the tyranny of flash mobs.

These challenges cry out for a relevant, persuasive, open-minded left. It is tempting to believe that the need for credibility blocks radicalism. That is the Corbyn/Sanders argument. In fact, the truth is the opposite. Credibility is the foundation of radicalism. If you are not credible on your intentions, your trustworthiness, your judgement, your character, your instincts, then your radical policies will never be given the chance to breathe. But establish that foundation, and there is a majority to be made. Just look at the new Liberal government in Canada.

There is a host of contingent and tactical issues that needs to be addressed by those active on the political stage. But the structural and strategic issues listed here are a matter for us all.

David Miliband, the president of the International Rescue Committee, was the Labour MP for South Shields (2001-2013). He writes here in a personal capacity

This article is part of a New Times collection of the future of the left. Read the other pieces here.