Will Chancellor Philip Hammond block customs union exit?

Theresa May wants to achieve three big things in her Brexit speech tomorrow.

She wants to give MPs the most important pieces of her jigsaw puzzle showing what kind of relationship she would like with the European Union - so that when they are asked to vote to give her the power to trigger Article 50 talks on leaving the EU they can't complain they are doing so blind (you will note that I and she assume the Supreme Court will rule against her and give Parliament the final say on whether Article 50 can be triggered by her).

Second she wants to give businesses as much certainty as she can about their future trading relationship with the EU.

So she will confirm that the UK will withdraw from formal membership of the single market.

But she also wants to negotiate the best possible equivalent trading relationship with the EU.

And the PM would like to announce that we will quit the customs union, the arrangement that dispenses with border checks on intra-EU goods trade - because failure to do so would prohibit us doing our own trade deals with the likes of Trump's America (and render utterly fatuous today's headlines saying Trump is gagging to do a free trade deal with us).

But the Chancellor has not yet been persuaded of the need to quit the customs union. And I am told she will not announce we're out of the customs union until Philip Hammond is on board (which he may be by tomorrow morning - but that is not certain!).

Finally she wants to make it clear to the other 27 EU government heads that they don't hold all the cards in the negotiations.

That is why Hammond said yesterday in Germany's Die Welt that if the EU screws the UK on trading arrangements he'll slash corporate and other taxes to attract businesses and talent away from the continent to Britain (the French are thought to be especially concerned about having the UK as a de facto Singapore on their doorstep, given that there has already been such a brain drain from Paris to London).

High stakes stuff! It is certainly the most important speech May has ever made, and may ever make. No pressure then.