Boris Johnson's new Brexit offer to the EU comprehensively rips up the backstop agreed by Theresa May - but it contains one proposal that may upset some Brexiter purists, namely that Northern Ireland should more-or-less remain in the single market for goods, food and agricultural products, subject to rules set by Brussels.

At 3pm this afternoon Johnson sent a four page letter to the president of the European Union commission, Jean-Claude Juncker, outlining his plan to break the Brexit impasse.

Along with the letter, Johnson's negotiator David Frost will present a six page explanatory note, and 40 pages of legal text to replace the controversial Northern Ireland protocol in the Withdrawal Agreement.

The main elements of Johnson's plan are:

1) There should be "an all-island regulatory zone on the island of Ireland, covering not just sanitary and phytosanitary and agri-food rules, but all goods, thus eliminating regulatory checks for trade in goods between Northern Ireland and Ireland.

2) This new regulatory zone will in effect create a new border between Northern Ireland and Great Britain.

Goods, livestock and food moving from Great Britain to Northern Ireland will have to be checked to ensure that they meet EU regulatory standards.

3) Brussels will continue to be in charge of standards of quality for goods and food in Northern Ireland.

And because this means the people of Northern Ireland will continue to be subject to laws made in Brussels, when the rest of Great Britain will not be, this arrangement will have to be approved either by the devolved assembly in Stormont, if that devolved government is ever up and running again, or via some other "democratic" mechanism such as a referendum.

4) Approval by Stormont or referendum would be needed six months before the new all-island goods and food market is created, currently scheduled for the beginning of 2021.

And the consent of the Northern Ireland people, either via Stormont or some other means, would have to be re-obtained every four years thereafter.

5) The customs union will have no place in the UK or Northern Ireland alone, either as backstop, a temporary arrangement or permanently.

6) There will be some "customs" friction on goods flowing across the border, because Northern Ireland (and the whole of the UK) will leave the customs union, never to return.

But Johnson believes most customs tracking and checking can be done in firm's own warehouses and other premises, with no need for new buildings or infrastructure to be built - and thus honouring the EU's insistence there should be no such physical infrastructure.

7) Northern Ireland will follow the EU's customs code, to give confidence to the EU that all necessary customs checks are being made on goods, food and livestock that passes back and forth between Northern Ireland and the Republic of Ireland.

Here the pros and cons of what Johnson is offering:

1) Northern Ireland's DUP, whose votes in Parliament Johnson needs, saw the plan last night and seem to be persuaded that its sacred principle that Northern Ireland must remain properly integrated into the UK is protected by the stipulation that democratic consent must be re-obtained ever four years.

2) The EU will be anxious that Northern Ireland's politicians will have the ability to blow up the arrangement every four years.

3) Dublin and the EU will be concerned that the new customs plans may not quite provide the frictionless border they say is implicit in the Good Friday Agreement.

4) But if the EU is prepared to be flexible and imaginative, what Johnson is offering just about meets its main requirements, namely the integrity of its single market, the preservation of an all-Ireland economy and the avoidance of a hard border.

5) That said the EU will argue that what Johnson describes is a potentially desirable end state, though it will be concerned that end state may take years to arrive.

It will says there is no guarantee that the new customs arrangements will be functioning and operable by the end of 2020, when - if a Brexit deal is agreed at the last - transitional arrangements to keep the whole of the UK in the single market and customs union come to an end.

This transition can be extended to the end of 2021, but EU leaders may feel even that is not long enough.

They will want a legally enforceable guarantee from Johnson to keep the border open and protect their single market for just as long as necessary till the new arrangements are seen to work - and Johnson is offering no such guarantee.

6) Johnson is offering almost nothing to woo Labour MPs who would prefer a Brexit deal to no deal or a referendum.

In fact he will alienate many by confirming that all he wants for Great Britain after the transition is a fairly basic free trade agreement with the EU, rather than the closer relationship they would prefer.

Will the EU reject Johnson's offer peremptorily and out of hand. I very much doubt that.

Will they engage in detailed negotiations to turn what he is offering into text they can accept?

That is harder to predict but I doubt it. By the middle of next week, Johnson may well have abandoned the negotiations and be concentrating exclusively on no-deal preparations.