When President Macron meets Theresa May at the Franco-British summit at Sandhurst, the elite military academy, on Thursday, the vital “take away” for him is that Brexit is not a done deal. It can, and quite possibly will, be reversed by an increasingly likely referendum on May’s Brexit terms early next year – once the Brexit terms are clear, but before Britain is due to leave on 29 March 2019.

It is essential, therefore, that Macron – who believes passionately in a strong and united Europe – continues to express his heartfelt support for continued British membership of the European Union, should that ultimately be the will of the British people.

There are some in Brussels – and Paris and Berlin – who think there might be rich commercial pickings from Brexit. Macron and Angela Merkel rightly understand that the ill effects of European fragmentation and division far outweigh any immediate “beggar thy neighbour” advantages of EU nations gaining short-term business at the expense of Brexit Britain.

Why I am I so confident that Brexit can be halted?

When Theresa May became prime minister 18 months ago, she called for national unity behind the narrow Brexit majority of the June 2016 referendum. An unenthusiastic remainer from the outset, she morphed overnight into to a hard Brexiteer, foolishly hoping to appease the right wing of her party, and Nigel Farage of the populist Ukip insurgency, by supporting the idea of leaving the European customs union and single market.

She failed to obtain that unity. “Hard Brexit” alienated not only the British business community, but swathes of moderate voters and almost all young people who fiercely oppose a “fortress Britain” which limits their ability to live and work across Europe. It also led to a crisis with the island of Ireland because of the near universal determination, in both the Republic of Ireland and the political parties in Northern Ireland, to avoid a “hard border”.

In an attempt to regain the initiative, May called an early election last June. This completely backfired. In a dramatic turnabout determined in large part by younger voters, many of whom had not voted in the referendum, May failed to win the election and the opposition Labour party made big gains. Under the leadership of Jeremy Corbyn, Labour has been reluctant to commit to a firm position on Brexit, but the voters – especially the young – correctly believed Labour to be opposed to hard Brexit and in its “heart of hearts” a pro-European party, and this was a significant factor in the election outcome.

Opinion polls have for months now shown a majority against Brexit and there has been a surge of support for continued EU membership among young and middle-aged people. The costs of Brexit have become apparent, with research suggesting that the value of Britain’s output is now around 0.9% lower than was possible if the country had voted to stay in the EU, which equates to almost exactly £350m a week lost to the British economy – an irony not lost on those who backed leave because of the claim made on the side of the infamous bus.

There has also been a change of tone in national media coverage of Brexit. Until recently, Brexit was regarded as inevitable and the only issue was what terms Britain would secure from Brussels. Now Brexit is regarded as far from inevitable, and commentators have started pointing up the reality that, while Britain obsesses about Brexit, our European partners are going about their business of forging a single market in services and integrating their trade and economies intensively.

A further important new theme is the “opportunity cost” of Brexit in terms of the distraction it represents from the real challenge facing Britain – namely, the social crisis afflicting the poorer cities and regions of the country, which voted heavily for Brexit in the referendum as a protest for being neglected by the elite in London.

As the son of a poor Cypriot immigrant into Britain, I understand this profound sense of alienation. Having become a parliamentarian, and therefore part of the “establishment”, I admit our collective failure to understand the plight of those in this country who didn’t go to university and get well-paid jobs, and whose children are faced with declining living standards. We have let them bear the brunt of the rebalancing act between the west and China and the developing world which we call “globalisation”. It is a big mistake to patronise those who voted for Brexit as if they were ignorant or misinformed. Today’s social crisis is ours to own, with humility.

The imperative, I believe, is for a radically reforming government in the tradition of Clement Attlee, Britain’s great postwar social democratic leader. It needs to work tirelessly to eradicate the inequalities magnified by the deregulation of the Thatcher era which, in key respects, persisted under Tony Blair and his successors. Britain is facing a crisis in housing, education, healthcare, employment and the incessant rise of social and regional inequalities. All this is feeding populism and undermining the fabric of our nation.

I resigned last month as head of the National Infrastructure Commission in order to help create an agenda for a radical “Attlee mark two” government. I believe the British parliament and people can be persuaded to ditch Brexit before “D-day” of 29 March 2019, but it will only come from a new settlement at home.

In the coming months, I will be touring Brexit strongholds around the country, not to convince but to listen and learn. Out of this, I hope, will come a programme to get Britain moving, as a credible antidote to Brexit and the scapegoating of “Brussels”and foreigners which underpins it.

There are still 15 months until Britain’s departure from the EU, time enough to hold a referendum on the government’s proposed Brexit terms. I am confident that the British people, provided with a credible and ambitious social plan, will recognise that the balance of advantages lies in continued EU membership.

The Brexit nationalist spasm can be stopped. But this is a time for humility, purpose and courage, not insults. And we need our European friends and allies to take the same approach.

• Andrew Adonis is a Labour peer, former transport secretary