Britain's relationship with the United States and our membership of the European Union have been the fundamental building blocks of our foreign policy but today we risk being less relevant in both. Europe is engulfed by the eurozone crisis and the US, weary after ten years of war in Afghanistan, is rebalancing its priorities from the Atlantic to the Pacific. That broader context only makes the Prime Minister's decision to leave Britain more isolated than at any time in the 38-year history of our EU membership even more dangerous.

The recent EU summit could and should have taken the vital decisions needed to stabilise the eurozone and boost growth and jobs but, instead, it was economically inadequate and politically damaging. There was no plan for growth agreed, no credible plan for reducing deficits agreed, no plan for recapitalising the banks agreed and no plan agreed for the European Central Bank to act as the lender of last resort. That the British government did so little to advance these objectives is inexcusable.

There was hardly any evidence of either the Prime Minister or the Foreign Secretary visiting European capitals ahead of the summit to build support for Britain's position. Indeed, the government's demands were tabled only a few days before, after David Cameron's mauling at the hands of his backbenchers at Prime Minister's Questions in the House of Commons.

Why did the Polish foreign minister, Radosław Sikorski - potentially a key ally for Britain - the week before the summit single out the UK for criticism in a speech and accuse the government of failing to provide political leadership on Europe?

Elephant in the room

To win the leadership of his party, Cameron promised to pull out of the centre-right grouping the European People's Party, as he did, and so he was left unable to attend the pre-summit meeting of the leaders of France, Germany, Portugal, Ireland, Finland, Bulgaria, Malta and Poland. When we entered the final hours of the summit in so shambolic a fashion, is it any wonder that Cameron was left unable to secure a single objective Britain had set or secure a single ally?

We have heard a lot about "vetoes" but to veto something means to prevent it from happening. Cameron walked out having prevented nothing from happening and having failed to secure any of his demands; that is not called a veto - that is called defeat.

Isolation can sometimes be a price worth paying for getting your own way but isolation achieving only defeat is unforgivable. Despite all the talk about protecting the City, the Chancellor was unable, 24 hours afterwards, to point to a single piece of financial regulation that was now not going to be applied to Britain as a result. Instead, we've got up to 26 countries discussing financial services without Britain being at the table, a development John Cridland of the Confederation of British Industry described as "the elephant in the room".

The roots of what happened on the night of Thursday 8 December lie deep in Cameron's failure to modernise the Tory party. Just because he puts party interest before the national interest, there is no reason others should do the same. That is why I make a genuine offer to Liberal Democrats to work with us to try to get a better outcome for Britain, between now and when this agreement is likely to be finally tied down in March. Work can and should start immediately both to win back friends and allies and to consider what rules and procedures can avoid Britain's further marginalisation.

My message to Lib Dems would be that, over the next few years, the public will reward politicians who show serious statesmanship, not shrill showmanship in the face of economic events none of us has witnessed before and the outcome of which remains uncertain.

This is the immediate task. But over the longer term, we must also remake the case for British membership of the EU. Today, according to one ICM poll, 49 per cent would vote to get Britain out of Europe, against just 40 per cent who would prefer to stay in.

What are the reasons for this and what should a progressive response be? In a recent speech, I talked about how, to my parents' generation, the rationale for Europe was establishing peace and stability on the continent after a century scarred by two world wars. This was a cause that had powerful emotional resonance. However, for the 20 years after Britain joined the European Community, that emotional cause was supplemented by a somewhat drier one: that being part of Europe would help reverse Britain's postwar decline and would help boost our prosperity and productivity.

Riding the tiger

Britain's rising prosperity during the long boom that began in the 1990s contributed to a growing sense of national self-confidence, which again led people to question Europe's role. One response to this rising scepticism, however, not only failed but, certainly in this country, actually heightened suspicions about the intentions of Europe's institutions. The push for anthems, flags and the apparent aping of the symbol of nationhood left the impression of a half-built superstate and provided a rallying point for Europe's opponents.

In response, a defence of the status quo won't be good enough. I do not believe Britain would ever be a "pygmy" nation but I believe we are better off as part of a market of 500 million people, with a £10trn economy.

In recent days, in Durban, South Africa, we saw welcome progress in global climate talks. Whether on climate development or trade, Britain's voice is amplified on the world stage by our European membership. Yet our future in Europe cannot be taken for granted. Cameron has embarked on a very dangerous course. He has closed his eyes and bet that he can ride the Tory eurosceptic tiger. The rest of us should open ours and make the case for a reformed Europe before it is too late.

Douglas Alexander is shadow foreign secretary