Brexit is unusual as a game of poker, in that one side folded long ago but has still not revealed its losing hand. For months, the EU has insisted that Theresa May’s only options for a deal would lead to either a soft Brexit for the whole UK, or a sea border between Great Britain and Northern Ireland. For months, critics have challenged the government to spell out which of these two ostensibly intolerable concessions it intends to make. Now it seems we know. The prime minister will concede both.

Capitulating to Brussels will be the easy part. After that, May will have to lie to the hard Brexiters, bully the Tory remainers, and call the bluff of the Democratic Unionist party. As the Brexit circus enters its final month, here is its tightrope.

First, Brussels. The EU’s offer springs from its immutable and non-negotiable red lines: to preserve the single market, the Good Friday agreement, and Ireland’s invisible border. Only two outcomes can satisfy all those requirements: the whole UK remains in the whole single market and customs union, or Northern Ireland stays in the customs union and single market in goods while Great Britain diverges.

Brexit: May faces cabinet backlash over indefinite customs union Read more

May has decided to mix and match those outcomes. It appears the whole UK will remain in the customs union, so there are no tariff divergences or checks either on the island of Ireland or within the United Kingdom. And Great Britain will leave the single market, thus necessitating “de-dramatised” regulatory checks on goods crossing the Irish Sea.

May’s surrender is not in doubt. Neither is the resistance to this deal from all opposition parties. Consequently, the prime minister’s only task is to fool or blackmail her MPs into supporting it.

Her most pressing duty will be to hoodwink the parliamentary hardliners in thrall to Boris Johnson and Jacob Rees-Mogg. May will attempt this ambitious deception principally by insisting that the permanent customs union will in fact be temporary. It will not.

The Irish backstop is designed to be all-weather and for all time. It ensures that no hard border on the island can emerge at any point in the future. For this reason no side can time-limit it or unilaterally dissolve it. The government may well pretend that technological solutions will eventually develop or that it can easily terminate the agreement. This will be a lie. The EU has made clear that the only customs arrangement which enables an invisible border is the customs union. Northern Ireland will not leave it and plausible alternatives will not emerge. That means the EU will continue to set the UK’s tariffs, and the UK will never sign meaningful trade deals with other countries.

This leaves May with two questions. First, are the hard Brexiters gullible enough to believe her? If not, will they vote for a Brexit with the customs union if the alternative could be a people’s vote and no Brexit at all? In her conference speech the prime minister threatened the purists with no Brexit if they failed to “come together”. They must now decide.

When it comes to the DUP, the prime minister will not need to lie. On the contrary, she must be honest about her priorities for the first time. The truth is that May’s aversion to migration trumps her unionism. She may theoretically value the UK’s economic integrity, but she demonstrably loathes the free movement of people. If given a choice between continuing free movement with the EU or increasing checks across the Irish Sea, she will choose the latter.

The more interesting question goes to the DUP. Which do they consider the greater existential threat: increased regulatory checks on goods travelling from Great Britain, or a government led by Jeremy Corbyn? May is banking on the latter. She may attempt to bribe them. She may simply let them press their nuclear button and hope that enough Brexit-minded Labour rebels come to her rescue.

Now Brexit really is threatening to tear the UK apart | Martin Kettle Read more

May’s third target is the most overlooked: Tory remainers. Former home secretary Amber Rudd has warned that dozens of her colleagues will vote against a Canada-style deal. The arrangement currently being prepared equates to Canada plus customs union, and delivers less integration than the Chequers proposal. That is to say, Great Britain will no longer harmonise its regulations on goods, which jeopardises jobs and growth. The deal also ignores services, which accounts for the lion’s share of the UK economy. The financial services sector, in particular, has warned of significant damage.

If the prime minister cannot blackmail these remainers with the threat of no deal, she may attempt to woo them with innuendo. The political declaration on the future relationship could, for example, hint at a future full single market relationship if the UK (though certainly not under the stewardship of Theresa May) softens its red lines. She may succeed; but she should not underestimate the challenge.

As befits such an undignified poker game, May has become a desperate gambler. In order to secure a withdrawal agreement, she must win over disparate parliamentary groups campaigning for radically different agendas. If she cannot subdue them with carrots, sticks or outright lies, she will dare each to defy her. In her recklessness, the prime minister plays her final hand not with poker chips, but people’s lives.

• Jonathan Lis is deputy director of the thinktank British Influence