The British cabinet's endorsement of a draft withdrawal agreement and outline of a political declaration is, as Theresa May said on Wednesday night, a decisive step in the process of leaving the EU.
The agreement represents a compromise that will leave the UK within a customs union with the EU and subject to many of the same regulations until a comprehensive new economic relationship is agreed.
It leaves open the possibility of extending the transition period, when Britain will follow all EU rules including those on the free movement of people, beyond December 2020. And it envisages a long-term economic relationship based on tariff-free trade and a level playing field on regulation, state aid rules and environmental and labour standards.
The Northern Ireland-specific backstop has disappeared from view but it lies concealed beneath the UK-wide customs union with the EU. Northern Ireland will remain in the same customs territory as the rest of the UK, but it will be subject to the EU customs code and to some EU regulations.
Britain will not be able to end the backstop unilaterally and its governance will fall under the same structure as the rest of the withdrawal agreement, with an arbitration panel made up of two representatives from Britain, two from the EU and one independent member.
The protocol on Ireland is, despite its reassuring language about the integrity of the UK, unacceptable to the DUP. Conservative Brexiteers are so angry with the agreement that they are plotting the prime minister’s overthrow – an endeavour that meets with the warm approval of the DUP.
Pro-European Conservatives share the Brexiteers' view that the deal represents the worst of both worlds by tying Britain to EU rules after it has lost its seat at the table in Brussels. Remainers are willing to risk a no-deal Brexit by voting against it and Brexiteers are prepared to risk a reversal of Brexit if parliament rejects no deal in favour of a second referendum.
The prime minister will make the case for the deal in the House of Commons on Thursday, and business groups will reinforce her message that the agreement is the best way of protecting the economy after Brexit. But when she returns from a summit in Brussels on November 25th with the deal agreed, May will face a grim parliamentary arithmetic that offers no apparent route to winning a majority for the agreement.
If MPs reject the deal, Britain will face a political crisis that could precipitate an attempt to renegotiate it, a general election, a second referendum or most likely a no-deal Brexit, with catastrophic consequences for the UK and serious damage to Ireland.