If we learnt anything during the EU referendum, it is to take pro-leave predictions with a pinch of salt

The claim from Change Britain – the so-called “Clean Brexit” group – that Britain stands to gain some £450m a week from leaving the EU and its single market should be taken with a degree of scepticism.

It seems that the Vote Leave campaign, which has morphed into this euphemistically named pressure group for hard Brexit, has not changed its misleading ways. It is barely six months since they toured Britain with a bus bearing the misleading message that the country would soon have £350m a week extra to spend on the NHS after Brexit. This was soon abandoned, rescinded and ridiculed out of existence. This new, at best arbitrary, calculation deserves to meet the same fate.

Rather than easily observable closures of plant and measurable job losses – though there will be those too – the most likely consequences of Brexit will be a decades-long loss of investment, job creation and gains in living standards from thousands of decisions by businesses to invest elsewhere rather than the UK, including elsewhere in the EU.

Of course, as the former Governor of the Bank of England, Mervyn, now Lord, King remarked, the European project has in some respects been a failure. Yet he knows better than most that Britain remained outside the main failure, the single currency, and staying out of the euro with a floating exchange rate helped the UK make the very best of the single market, a British-driven project. While there will certainly be new trade deals and fresh investment from nations the UK needs to build links to, such as China and India, there is no guarantee that they will be able to make up for lost trade and investment from Brexit, and certainly not rapidly. To do so would require a shredding of protections for workers and consumers – not a price worth paying.

As the incoming Commerce Secretary in the Trump administration remarked this week, Brexit represents a “God-given opportunity” for rivals to steal business from the UK, especially in financial services, where upstarts from Frankfurt to Cyprus are already courting banks and others. The City of London has formidable strengths, but it has no God-given right to global or even European pre-eminence. When Britain boasted the largest shipbuilding, textile and motorcycle industries in the world there was a similar sense of complacency about British supremacy to that now emanating from the Leave camp about financial services. The car industry, aerospace and even the fishing industry, ironically, are also critically threatened by Brexit. As these hard facts come more sharply into focus there will be irresistible pressure to place before the people the terms of exit from the EU. There must be a parliamentary or popular vote to ratify this massive change, with a debate informed by a much closer idea of the alternatives. We will then see how much poorer we are likely to be, and the voters who did not vote for that on 23 June and some of those who those who voted Leave on 23 June will be able to decide whether their distaste for immigration is sufficient to justify a permanent reduction in living standards for themselves and their families.