Tony Blair's first visit outside London after his election landslide in May 1997 was to the Royal Ulster Agricultural Show in Balmoral, near Belfast. Among the prize bulls, he tried to reassure unionists that they were safe under a Labour government, and in his speech he said: ‘“None of us in this hall today, even the youngest, is likely to see Northern Ireland as anything but a part of the United Kingdom.” It worked. Now, after Boris Johnson’s Brexit deal, the unionists can no longer be so sure.

With the race to pass the legislation paused, we can stand back and judge the deal on its merits. For if there is to be an election it will be a referendum not on Brexit but on Boris Johnson’s deal. And this deal is a threat to the union — not the European Union but the continuation of the United Kingdom.

The vast majority of the text is identical to Theresa May’s deal, and the problems it will cause for our society, political system and economy have been repeatedly rehearsed.

Indeed, that was voted down three times in the House of Commons, including twice by Boris Johnson, and was judged enormously unpopular in opinion polls.

What is new is the removal of the UK from the Customs Union and the provisions on Northern Ireland. As the DUP plaintively points out, these have not been debated and nor is there any economic assessment of their implications for Northern Ireland. Nor has there been any proper consideration of the unintended consequences for the future of the United Kingdom.

The Northern Ireland measures are not what Boris Johnson wanted nor the result of a clever negotiating strategy. He proposed something completely different. He wanted a hard customs border between Northern Ireland and the rest of Ireland. That was what won him DUP support.

But at the last minute, in a panic to achieve a deal by the arbitrary date of October 31 he had set himself, he capitulated and accepted the EU proposal of a hard customs and regulatory border between Northern Ireland and the rest of the UK.

Thank goodness he did jettison his earlier ideas because a hard border in Ireland would have posed an existential threat to the Good Friday Agreement (GFA). But he should have thought more carefully about what he was putting in its place. It is clear from his answers in the House of Commons this week — where he falsely denied that Northern Ireland business would have to fill in EU forms to send goods to the UK — that he has no idea what he has agreed to. A grasp of detail is not his strong point.

The Northern Ireland peace process is a carefully balanced seesaw. What Johnson has done is leap from one end of the seesaw to the other, disrupting that balance

The Northern Ireland peace process is a carefully balanced seesaw. What Johnson has done is leap from one end of the seesaw to the other, disrupting that balance. And the implications for the future of the UK are serious. A border in Ireland would have been a threat to the identity of nationalists in Northern Ireland as Irish.

A border between Northern Ireland and the rest of the UK is a threat to the identity of unionists as British. They legitimately fear that this is the beginning of a slippery slope to a united Ireland, and that is why we have once again started hearing worrying noises from the Loyalist paramilitary groups. As the Chief Constable of Northern Ireland warned last night, this could lead to civil disorder or worse.

The new border Boris Johnson has created is not just wider than he claims — and certainly not a transitional arrangement as suggested — but will grow wider over time as the UK diverges from the EU in terms of regulation and tariffs. More and more goods will be put on the list of those that need to be checked. The problem is not just for business but the very idea of a border separating unionists from the country they want to be united with.

The UK Government is obliged under the GFA to hold a border poll if there appears to be a majority for a united Ireland. The numbers are already moving in that direction as a result of Brexit, as Catholic voters are forced to choose between continuing in the EU and staying in the UK. Those numbers will continue to move as a result of demography and continued incorporation into the single market and customs union.

The impact of all this on Scotland is obvious. First the SNP government is bound to demand the same treatment as Northern Ireland, which is going to enjoy a soft Brexit while Scotland will face a hard Brexit, despite voting in broadly equal proportions to remain. And when Boris Johnson denies them this, as well as refusing a further referendum, he will add to their list of grievances and drive up support for independence still further, which has already risen to 50 per cent in recent polls. So, the one thing the Johnson deal will do which the May deal did not is set out a plausible path to a united Ireland and an independent Scotland. Is that really what English and Welsh Brexit voters intended? I don’t think so.

If you feel strongly about the UK, the election may be your last and only opportunity to vote to stop its destruction

Of course, the best way to decide this question would be in a further referendum rather than an election which will mix Johnson’s deal with other issues, like the unpopularity of Jeremy Corbyn. But if you live in England and Wales and feel strongly about the continuation of the United Kingdom, this election may be your last and only opportunity to vote to stop its destruction. Because afterwards the only people who will be able to vote on it are those who live in Scotland or in Ireland.