Today Johnson and Sunak have written a joint article in "The Sunday Times" that the NI rise is will take place.

This is an important statement. Johnson, of course, cares little whether it goes ahead and will be very aware of the large number of Tories who want to scrap it. If he thinks that will benefit him, he will scrap it.

But Sunak was already tied into going ahead with it by rumour, and has now made that his position in writing.

So we have reached yet another point of instability. If Johnson does backtrack, Sunak may resign and we are into an open contest for leader. But does he want to do that over an essentially unpopular tax? And if he does not, how diminished is his authority? It is a finely balanced decision.

Equally, though, does Johnson want to trigger that contest with Sunak? Probably not.

So at the moment, the tax stays. But it could change in days and what then happens then is extremely hard to predict. The options are clear, but which one will be taken I could not guess.