From the beginning of my conversations on Brexit with the Labour leadership, I advocated a “radical Remain” stance: the UK should be for the EU, but not this EU. The labour movement in Britain and elsewhere has been critical of the operations of the state at the national level without calling for its disbandment. Similarly, we can be critical of the EU without seeking to leave it. We can expose the failings of its institutions and be fierce opponents of its neoliberal economic doctrine without wishing for the break-up of the bloc.

I warned Jeremy Corbyn and John McDonnell that, after almost four-and-a-half decades of entanglement with Europe, it would be very hard for the UK to leave even if it wanted to. “Never would you be more dependent on the EU than after Brexit,” I said. The UK would expend huge amounts of economic and political capital in pursuing withdrawal.

In the end, Corbyn’s Labour Party accepted the radical case for Remain, though I wish that this was put forward more purposefully during the 2016 referendum campaign.

After the Brexit vote, the Democracy in Europe Movement 2025 (DiEM25), which I co-founded, debated the options. One idea was to campaign for a second referendum to reverse the result. Another was to accept Brexit without qualification and concentrate on the economic policies to be enacted in a post-EU Britain. The final option, which won the DiEM25 internal vote, was to support a “transitional period” of five to seven years, and it is this that Labour is edging towards adopting. A radical transition could be a period in which both the UK and the EU become something altogether new, and something more progressive.

As democrats, we must accept the result of the referendum. To seek to overturn it through a second vote would be to do to the British people what the EU did to the Irish over the Lisbon Treaty: keep repeating the question until they delivered “the right answer”. The UK electorate would rightly conclude that referendums only count when they deliver the result that the establishment wants.

The promise of a second referendum, as my Brexiteer friends (such as the former Conservative chancellor Norman Lamont) have noted, would also give the EU a strong incentive not to yield on any front. After this, Britain would be left to go cap in hand to the EU, begging to be readmitted, without any of the concessions that Margaret Thatcher and others secured.

The referendum was a binary choice. It said nothing about what kind of Brexit should be pursued. I agree that full sovereignty should be restored to the House of Commons, but that means MPs must have a mandate to debate the model of Brexit adopted.

There should be an interim agreement between the EU and the UK, so that parliament has a full term in which to deliberate. I warned Corbyn and McDonnell that the two-year withdrawal period after the triggering of Article 50 would prove insufficient. Now Labour has proposed that the UK should remain in the customs union and single market for between two and four years (though longer would be preferable).

Were the UK government to propose such a transition, Michel Barnier, the EU’s chief negotiator, would not be able to refuse. During this period, Britain would accept the supremacy of the European Court of Justice and continue to make budget contributions. Angela Merkel – who is, in effect, the only national leader who matters in Europe – would breathe a sigh of relief, with her eventual successor as German chancellor given the task of agreeing the final Brexit deal.

Labour has left open the option of permanent membership of the single market if the EU grants the UK new controls over free movement. But this won’t happen. Nor should it. Borders should be an enemy of all progressives: either we are internationalists, or we are not. Those of us who have been critical of neoliberal globalisation have always pointed out that we have the free movement of goods, commodities and capital – but not of people.

It’s a sad day when the Labour Party is defending the end of free movement and, indeed, promising its end. But where Labour can be flexible and innovative is over the question of social security. At present, European companies are able to import cheap labour and undercut domestic workers, who enjoy greater rights. Progressive parties should challenge the EU to promote solidarity and equality of working conditions for all European workers.

Labour’s performance at the 2017 general election showed what can be achieved if you ignore the free marketeers, the focus groups and what passes as “sensible” politics – and speak from the heart. Jeremy Corbyn and John McDonnell must now turn the debate about the EU into one about the British constitution, which remains an embarrassment.

Tony Blair’s half-hearted reforms left the UK in a constitutional vacuum. The absence of an English parliament contributed to the disenchantment of the pro-Brexit areas, especially in the north of England.

By proposing a genuine transitional period, Labour has shown greater political initiative and seriousness than Theresa May. To maximise this advantage, Corbyn and McDonnell must continue to develop an alternative vision of Brexit.

Yanis Varoufakis is a writer and economist and the former Greek finance minister

As told to George Eaton