Early next year, Britain will switch from a pro-Brexit to an anti-Brexit country. To be more precise: if not a single voter in the referendum two years ago changes their mind, enough mainly Leave voters will have died, and enough mainly Remain voters will have reached voting age, to wipe out the Leave majority achieved in June 2016.

This is the clear conclusion from a new YouGov survey for the People’s Vote Campaign. They show that demographic factors alone are causing the Leave majority to shrink by around 1,350 per day, or almost half a million a year. Crossover Day, when Remain moves into the lead, will be January 19. By March 29, the day the UK is due to leave the European Union, the Remain majority will by almost 100,000 – again, assuming that nobody who voted two years ago has changed their mind.

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This means that by March 29, it will be difficult to sustain the argument that the settled view of the British electorate is that Brexit should take place. We are told that we should “respect the verdict of the people”, and not reopen the decision they – we – reached in 2016. The latest research shows that this depends not only on the proposition that voters cannot change their minds, but on a specific definition of “the people”. It includes those who have died since the referendum – and excludes almost two million new voters who were too young in 2016 but will be old enough to vote by next March.

Here is how I worked out that January 19 will be Crossover Day.

Around 600,000 Britons die each year. Polling research has shown consistently that elderly voters backed Brexit in the referendum by two-to-one. We also know that they turned out to vote in larger-than-average numbers.

If we assume a turnout of 80 per cent among people who have since died, then each year the number of voters who die is around 480,000; applying the two-to-one Leave-Remain ratio, we can estimate that 320,000 pro-Brexit voters and 160,000 pro-EU voters die each year.

Now to new voters and the latest YouGov research. Within the past fortnight, YouGov has questioned 1,645 adults under 25, of whom 450 were too young to vote in 2016. Among those who have become 18 since Jun 2016, 65 per cent say they are absolutely certain – on a scale from 0 to 10 – to vote in a fresh referendum. I have counted only these people: if I were to include all new voters who express an opinion, the decline in the Leave majority would be even faster, and Crossover Day even earlier.

The number of people reaching voting age is 700,000 a year. A 65 per cent turnout implies an extra 455,000 young voters each year. YouGov’s poll finds that as many as 87 per cent would vote Remain, and 13 per cent Leave. It is worth pausing for a moment to reflect on those numbers. On an issue that splits Britain down the middle, we have a demographic group that divides by seven-to-one in favour of one side. I have seldom seen such a gulf between a particular group and the electorate as a whole.

Back to the calculation. YouGov’s figures suggest that each year 395,000 Remain voters and 60,000 Leave voters join the electorate.

Let us bring the two sets of figures together.

The annual change in the number of Remain voters: add 395,000 people reaching voting age; subtract 160,000 voters who die. Net effect: an extra 235,000 Remain voters

The annual change in the number of Leave voters: add 60,000 people reaching voting age; subtract 320,000 voters who die. Net effect: 260,000 fewer Leave voters

With the Remain vote rising by 235,000 a year and the Leave vote falling by 260,000 a year, the overall effect is to reduce the Leave majority by 495,000 a year. This works out at 1,350 per day. (The precise figure is actually 1,356, but I have rounded it down.)

In the 2016 referendum, Leave enjoyed a majority of 1,269,501. If that majority falls by 1,350 per day, it disappears 940 days after June 23, 2016. That is January 19, 2019, ten weeks before the UK is due to leave the EU.

To repeat: those calculations assume that not one person who voted in the last referendum has changed their mind – and that those whom abstained then would stay at home again. In fact, YouGov’s research shows that a fair number of today’s under 25s who were old enough last time but did not do so, would vote in a new referendum; and these people would divide 82-18 per cent for Remain. (This compares with a 75-25 per cent preference for Remain among the under 25s who DID vote last time.)

I have NOT counted any rise in turnout among the under-25s in my prediction that Crossover Day will be January 19. Were I to do so, this would, again, bring the day forward; indeed, it might already have happened.

YouGov also find that young voters are especially keen on a new popular vote. Those too young to take part in the last referendum divide three-to-one in favour. Among those who say they would definitely vote, the margin is more than four-to-one. These figures are not surprising but they are significant. We can debate the wisdom of allowing those who were old enough to vote last time to have second thoughts. New voters, however, are not requesting the right to have second thoughts; they want their first chance to take part in a decision that could affect the rest of their lives.