I’m struck, as the British parliament moves towards the endgame on Brexit, with the number of times Australia, Canada, New Zealand and India have been advanced by the Brexiteers in the public debate as magical alternatives to Britain’s current trade and investment relationship with the European Union. This is the nuttiest of the many nutty arguments that have emerged from the Land of Hope and Glory set now masquerading as the authentic standard-bearers of British patriotism. It’s utter bollocks.

If Britain proceeds with giving effect to what future historians will legitimately describe as the longest suicide note in history by leaving the union, the cold, hard reality is that the mathematics simply don’t stack up in terms of credible economic alternatives to Europe. Much as any Australian, Canadian and New Zealand governments of whichever persuasion would do whatever they could to frame new free-trade agreements with the UK, the bottom line is that 65 million of us do not come within a bull’s roar of Britain’s adjacent market of 450 million Europeans.

For its economic self-interest, as well as the wider political interests of the west, Britain should remain in the EU

As for India, good luck! India’s trade and commerce bureaucracy is the most mercantilist and outright protectionist in the world. They virtually single-handedly sank the Doha round in 2009. In the same year, as prime minister of Australia, I launched a free-trade negotiation with Delhi. But a decade later, those negotiations remain at a standstill. The Australian economy is only 50% the size of Britain’s. A substantive India-UK FTA is the ultimate mirage constructed by the Brexiteers. It’s as credible as the ad they plastered on the side of that big red bus about the £350m Britain was allegedly paying to Brussels each week. Not.

So as a former chair of the Commonwealth ministerial action group, it’s my melancholy duty to report that the idea the old (or for that matter newer) Commonwealth could possibly substitute for Britain’s current economic arrangements with Brussels is an illusion.

Then the Brexiteers turn to that other great economic chimera, the US, as final salvation. The irony is that the current wave of nationalism, isolationism and protectionism that they so ruthlessly exploited in the Brexit referendum has also been unleashed in the US. Donald Trump’s America has little, if any, room in its political imagination for maintaining existing bilateral free trade agreements (see Nafta and Korea), let alone negotiating mutually advantageous new ones. Let’s not forget the very first act of the Trump administration was to withdraw from the Trans-Pacific Partnership, despite the economic damage done to three of its closest allies – Australia, Canada and Japan. If No 10 still thinks its “special relationship” with the White House will uniquely be capable of battering down the doors of a newly protectionist America, then good luck with that one too.

Facebook Twitter Pinterest ‘A substantive India-UK free trade agreement is the ultimate mirage constructed by the Brexiteers.’ Photograph: Victoria Jones/PA

In Australia, we have our own Land of Hope and Glory set who have been cheering the Brexiteers on to victory, as chaps would cheer on the home side at a good game of rugger on the playing fields of Eton. By and large, however, these Oz-Brexiteers have been confined to the right wing of the Australian conservatives and no longer represent the Australian political mainstream. They too, like the core of the British Brexiteers, are driven by a conservative political romanticism that we can all somehow go back to that ancient Arcadia of a white Anglo-Saxon world with “imperial preference”, all consummated by the solemnity of a Lord’s Test. Former prime minister Tony Abbott, for example, recently invoked the spirit of Wellington, Nelson and Drake in the pages of the Spectator in defence of the most ludicrous of all the Brexit possibilities – the no-deal, cold turkey outcome. Abbott’s message: the old empire is just champing at the bit for Britain to cut itself loose from all those continentals in Brussels. That too is bollocks.

For obvious cultural reasons, practically all Australians like Britain. They want Britain to do well. They are happy with the idea of allowing their kids to live and work in Britain, just as we welcome British kids in Australia. But with the best will in the world, we cannot turn the clock back to 1973 when Britain joined the EEC and told Australia to go carve out its economic future elsewhere. So we did – but following Britain’s example in Europe, we set about integrating our economy with our neighbours in the Asia-Pacific region.

Beyond Britain’s economic self-interest, there is a further reason why the UK needs to remain in the EU. It’s about the future of the very idea of the west itself, of western values and their wider contribution to the future of the international order. For centuries, Europe and the US have been the joint custodians of the western tradition – Judeo-Christianity, the Enlightenment and the political, economic and social freedoms of the current century. These in turn have helped shape many of the international norms underpinning the current order. However, the US’s future commitment to the very idea of the west, and western values in foreign policy terms, is now the subject of fundamental debate within the US body politic. It may prove to be a temporary, Trumpian aberration. But we would be foolish not to identify a new isolationism emerging.

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If the American pillar is looking a little shaky, Europe is looking even shakier. The strength of the political emergence of the far right across Europe’s four largest economies – Germany, France, Italy and Spain – is frightening. The European political centre is being hollowed out. All at a time when Europe’s economy is once again weakening. And that’s before you add the final blow to the solar-plexus in the form of Brexit. The bottom line is that a European Union without Britain will be a weaker international actor than it has been, particularly if the European centre of political gravity increasingly moves in a more populist direction. Without a strong Europe, the continuing idea of “the west” begins to look very weak indeed. And authoritarians around the world would like nothing more than a fully disembowelled west, no longer confident of what it actually stands for any more.

So my appeal to Brits, both Conservative and Labour, is to use this critical fortnight to start turning all this around. For Britain’s economic self-interest, as well as the wider political interests of the western community of nations, Britain should remain in the EU. Labour and the Conservative remainers should unite to defer the exit date beyond 29 March 2019. They should then support legislation for a second referendum – offering the British people a clear, informed choice between two tangible, concrete proposals: either voting for Theresa May’s deal, or for Britain to remain in the Union. That’s when I believe Britons’ native common sense, as well as their wider sense of international responsibility, would ultimately prevail.

• Kevin Rudd was prime minister of Australia from 2007 to 2010 and from June to September 2013