Most prime ministers arrive in Number Ten with a set of assumptions that will bring about their fall. They reach a peak that so many rivals ache to climb and yet begin unknowingly to clear the way towards their tragic end. This is the case with Theresa May in relation to Brexit. When she became prime minister in dramatic circumstances she chose to be trapped from the beginning. This week she must realise that all escape routes are blocked.

In fairness to May she faced the toughest inheritance of any prime minister since 1945. Brexit was the context of her sudden triumphant ascendancy and yet presented her with a nightmare. Even so at the start of a reign a prime minister has an authority he or she will only possess again if they win a big election victory.

After a chaotic, virtually insane leadership contest in the summer of 2016, the Conservative party had turned to her and she was ready to serve. She was way ahead in the opinion polls, which is almost as authority enhancing as winning a general election as MPs assume victory is on its way at some point.

In such politically potent circumstances May had some choices in relation to Brexit. She might have pledged to deliver the referendum verdict but to keep options open as to how this would be brought about. She might have insisted that the customs union and the single market were “on the table”, that cleverly evasive phrase used recently by a Labour leadership discovering the art of politics. The hardline brexiteers in her party might have hit the roof but she was newly enthroned and would almost certainly have remained in place to pursue her course. Perhaps she might even have dared to challenge their unyielding fantasies in a way that David Cameron had failed to do while reassuring them she would deliver Brexit.

Instead May chose to agree with the fantasies. She arrived in Number 10 with an entirely different set of assumptions than those that kept options open. One that has been largely overlooked is about to become pivotal.

An early May misguided assumption was first noted by Steward Wood, a former senior adviser to Gordon Brown at the Treasury. Wood wrote earlier last year that as home secretary May had easily opted in and out of some EU security arrangements. In her limited Home Office dealings with the EU she had her cake and ate it. Landing suddenly in Number Ten with no ministerial experience of economic, trading and industrial issues in relation to the EU, she assumed she could take the same approach with Brexit: full alignment in some areas, managed divergence in others, and a complete break in some sectors.

From the beginning the EU made clear the cake and eat it approach would be unacceptable but she and her then senior advisers assumed she could prevail. One of those former advisers, Nick Timothy voted for Brexit and was passionate about the need to leave the customs union. We know he was passionate because he has written with intensity on the theme since he left Number 10.

Liam Fox, May’s leadership campaign manager was similarly passionate. Evidently May was convinced by them. She closed the option on the false assumption that sunny uplands lay elsewhere. She also assumed that those she needed to woo were the Brexiteers although in the Commons, even before her misjudged election, there was a majority for a softer Brexit.

The obstacles she faces as a result of assumptions made at the intoxicating height of the early honeymoon are impossible ones. Perhaps she will win the vote on the customs union in the coming weeks by deploying words to cover over the cracks. She spoke of 'associate membership’ of the customs union in her Lancaster House speech last year. I am an associate member of a tennis club. I have never known what it meant except that it seems to involve having no access to a tennis court.

No doubt there will be more warm words this week about some form of customs arrangement. They will be meaningless as she and others have said their priority is UK trade agreements elsewhere, leaving a three course meal for the hope of a packet of crisps as one former senior civil servant put it yesterday. There will also be warm words from May on the need for a soft border in Ireland. They will be virtually meaningless too as all the key words are interpreted differently by half the cabinet, the Irish government and the rest of the EU.

Last December I wrote that May’s apparently triumphant Phase One Brexit agreement was similar to Tony Blair’s first UN resolution in advance of the war in Iraq. Every country could sign up to the first resolution because the words were deliberately vague. Blair never got a more precise second resolution. May is now seeking the equivalent of the second UN resolution. She is in the very dangerous phase where precision is required.

Blair arrived in Number Ten with assumptions about a Labour leader needing to be seen as tough on defence and as an unswerving ally to the US. His assumptions as he reached the peak of power led him weakly towards Iraq.

May moves towards her doom as she made choices that do no command majority support in the Commons, have no backing in the EU and offers no solution to the Irish question. She is dogged, conscientious, assiduous. Her enigmatic public presence, a lack of definition, keeps the show on the road. Her speech on Friday, like her previous two in London and Florence last year, will be carefully crafted and tonally diplomatic. Her genuine decency will be conveyed.

But what does she do when the EU rejects her approach? What does she do if pro-customs union Conservative MPs vote with their beliefs and her negotiating approach is defeated?

I do not know the answer to these questions, and nor does she, but they would not have been posed so starkly if May had arrived in Number Ten, a triumphant victor, with a different set of assumptions about how best to deal with her predecessor’s deadly referendum legacy. In her beginning she moved towards her end.