The penny seems to have belatedly dropped for Boris Johnson that he can have a no-trade-deal relationship with the European Union - what he calls an Australian relationship - and he can have Northern Ireland as a seamless member of the UK’s internal market.
But under the EU Withdrawal Agreement that he signed, he cannot have both.
Because if we trade with the EU under WTO terms, there will be highly significant tariffs levied on UK trade with the EU and highly significant fiscal and regulatory differences between the UK and EU.
And the default position in the Northern Ireland Protocol of the Withdrawal Agreement is that - absent agreement to the contrary between the UK and EU - all goods flowing from GB to NI are “at risk” of flowing across the NI border into the EU.
And therefore they would all be subject to tariffs at the border between GB and NI.
And goods flowing from GB to NI would be subject to verification that they are not designated as EU goods intended exclusively for the NI market.
This would be fracture of the UK in an economic sense. So little wonder that the PM wants to prevent it.
Except that his plan, to legislate via the Internal Market and Finance bills looks as though it breaches international law; that fear is apparently why Jonathan Jones, head of the government’s legal department, has quit today.
And if it is illegal, presumably the House of Lords will not hesitate to try and frustrate the legislation.
And anyway it is not clear the Withdrawal Agreement, approved by Parliament in this session, can be amended before a new Queen’s Speech.
And the EU won’t hesitate to enforce what it sees as the clear stipulations of the Withdrawal Agreement as a binding international treaty.
So this is one cake Johnson may not be able to have and eat.
His choice is an Australian relationship with the EU or a seamless UK, but not both.
Or perhaps there is a better way of looking at this...
Johnson’s negotiator Lord Frost seems persuaded that no trade deal with the EU is an excellent outcome.
But if he thinks that “threat" is a loaded slingshot pointed at Michel Barnier and the EU’s negotiators, he may find they see the catapult as pointing at his boss and the UK.