The devil will be in the detail of PM's 12-point Brexit plan

Not just a plan, but a 12-point plan for Brexit - soon to be delivered by the Prime Minister at Lancaster House, the seat of historic British diplomacy over Europe and the Commonwealth.

The rough outlines have been signalled - out of the jurisdiction of the European Court of Justice, out of the single market, and control of borders.

It will be termed a "clean break" by Brexiteers, and a "hard Brexit" by others - including the markets.

She will say: "We seek a new and equal partnership - not partial membership of the European Union, associate membership of the European Union, or anything that leaves us half-in, half-out."

Downing Street circulated the four principles driving the 12 points: certainty and clarity; a stronger Britain; a fairer Britain, and a truly global Britain.

But no details were given about these 12 points, and the devil will be in the detail.

:: Brexit terms explained - What you need to know

For starters, an exit from the EU customs union will be implied rather than explicitly stated. Why the need for constructive ambiguity here?

The Irish government is concerned about the impact on the border, and "rules of origin" arrangements are vital to multinational modern manufacturing supply chains for cars and planes.

Then, if the UK and the EU are about to forge a new "partnership", what will be the trade dispute mechanism?

No European Court of Justice is the claim, but what of the EFTA Court around the corner in Luxembourg? Or perhaps a separate EU-UK trade dispute court? Can that be negotiated in less than two years?

:: Ian King: Increasingly clear a 'hard Brexit' is on the cards

A transition deal of some sort will be mentioned to avoid a cliff edge in certain industries. The key is this: how long will this be in force for, and will this transition last into the next Parliament?

The risk is that financial services firms will now conclude that the Government does not want to keep the "passports" that enable trade across the EU, and they will activate their contingency plans immediately.

What is certain is that Mrs May will set out a strategy of reclaiming border control, and subject to that, maximum trade access.

It is also a strategy for a negotiation. Where it all ends up is not just down to the PM.