Under the terms of the Withdrawal Agreement, Northern Ireland will stay in the Customs Union.  And the rest of the United Kingdom will be a customs union.  This arrangement sets up a tug-of-war between the pro-Brexit and pro-Union instincts of most Conservatives.  As Brexiteers, they would be happy to see the rest of the UK leave its customs union, because that would help to maximise trade deals.  As Unionists, they would not, because doing so would threaten a customs barrier between Northern Ireland and the mainland.

This is why the Government has been indicating that it favours a “Stormont Lock” – in short, a guarantee that the rest of the UK will not leave its customs union without permission from Northern Ireland’s Assembly.  It has come down on the pro-Union side of the balance, which is right.

The fuss about additionally joining a customs union more or less identitical to the Customs Union is puzzling – at least, if those concerned are backers of May’s deal.  Sure, Greg Hands’ arguments against membership are as powerful as ever.  But customs union membership, as proposed by Ken Clarke earlier this week, would make no difference to the proposed status quo (if you slap a Stormont Lock on it, anyway).

In the last resort, the UK could always join a customs union and leave it.  What it can’t do is automatically leave the backstop, if the deal is agreed.  And it is precisely the backstop which locks us all, Northern Ireland and the rest of the UK alike, effectively into the Customs Union for as long as there is a Unionist-minded government in office.