There had never been any real doubt in Brussels that the EU’s proposal in February to, in effect, keep Northern Ireland in the single market and customs union after Brexit would “have a crash landing”.

In the days ahead of the publication of the legal text, the EU’s deputy chief negotiator, Sabine Weyand, had given indications to Olly Robbins, Downing Street’s senior Brexit adviser, of the bloc’s direction of travel on the problem of avoiding a hard border on the island of Ireland. But the actual document had not been shared with Downing Street. “There was no controlled landing on the UK side”, said one EU diplomat.

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The impact did not disappoint. The proposal would “undermine the UK common market and threaten the constitutional integrity of the United Kingdom”, the prime minister told the Commons. The Democratic Unionist party accused the EU of seeking to “annex” the province.

The aftershocks of that moment have left the Brexit negotiations on an uneven footing ever since and, even now, in the final stretch, it is not evident whether a compromise on this most thorny issue will emerge to allow things to stumble on.

Negotiators will formally slide back this week into what Brussels describes as “the tunnel”, a period of private talks, during which neither side is expected to brief outsiders about developments. “Once anything gets out, it melts like a snowflake,” said an Irish official.

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The British government is keen to make enough progress in the next few days for Donald Tusk, the European council president, to call an extraordinary Brexit summit of leaders for later this month, give the UK prime minister some momentum and have a deal safely sealed by December. This would offer parliament the necessary time to ratify the withdrawal agreement containing the £39bn divorce bill, provisions on citizens’ rights and the solution to avoiding a hard border on the island of Ireland.

A joint EU-UK political declaration will come along with it, giving a framework for the future trade talks. Discussions are taking place on finding the right language to allow May to claim that she is protecting the British economy from barriers to trade. But everyone knows there is really only one issue at hand.

The Irish border issue – or Gordian knot as Tusk described it – that has defined the Brexit negotiations was not burning red on the Brussels dashboard immediately after the 23 June 2016.

After that year’s summer holidays, as the EU started to put together the building blocks of its negotiating team, an internal “decision tree” was devised in the Berlaymont, the European commission’s headquarters in Brussels.

It was noted that a British decision on the customs union with the bloc would decisively shape the talks. But it was only in October, at that Conservative party conference, when Theresa May, under the advice of her then joint chiefs of staff Nick Timothy and Fiona Hill, told the faithful that she was taking the UK out of the single market, customs union and the jurisdiction of the European court of justice, that senior EU officials realised they had “a border problem”.

The first EU negotiating position came in April 2017, when May triggered article 50. The issue of avoiding a hard border would require “flexible and imaginative solutions”, it said. The talks on this issue centred on technological solutions away from the border.

But the then taoiseach, Enda Kenny, was getting nervous. EU officials started to brief journalists against the “magical thinking” coming from London.

By the autumn of that year, the Irish were pushing for what was the first time being described as a “backstop” solution should the technological solution not emerge. A “talking points” paper was circulated among the EU member states in October and it was obvious that trusted trader schemes and clever ways to minimise regulatory check solutions were not going to fly. “We have got a major problem,” the UK’s lead official on the issue told colleagues.

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Downing Street was feeling huge pressure to make progress last December, with the EU refusing to talk about the future trade relations until the opening withdrawal issues were settled. It was in this context that an EU-UK joint report was agreed in which the British government committed to a Northern Ireland-specific backstop while also insisting, following pressure from the DUP leader, Arlene Foster, on a clause saying there could be no border in the Irish sea. EU officials at the time described it as “a huge fudge”. “How in hell will this ever be put in a legal text?,” asked a senior official involved in the talks.

In London, the cabinet’s chief Brexiters, David Davis, Michael Gove and Boris Johnson, raised concerns but May’s chief of staff, Gavin Barwell, successfully assured them that the commitments were all but meaningless.

“I don’t know what it means but the civil servants tell me it’s fine,” said Downing Street’s director of communications, Robbie Gibb.

Quick guide Brexit and backstops: an explainer Show Hide A backstop is required to ensure there is no hard border in Ireland if a comprehensive free trade deal cannot be signed before the end of 2020. Theresa May has proposed to the EU that the whole of the UK would remain in the customs union after Brexit, but Brussels has said it needs more time to evaluate the proposal. As a result, the EU insists on having its own backstop - the backstop to the backstop - which would mean Northern Ireland would remain in the single market and customs union in the absence of a free trade deal, prompting fierce objections from Conservative hard Brexiters and the DUP, which props up her government. That prompted May to propose a country-wide alternative in which the whole of the UK would remain in parts of the customs union after Brexit. “The EU still requires a ‘backstop to the backstop’ – effectively an insurance policy for the insurance policy. And they want this to be the Northern Ireland-only solution that they had previously proposed,” May told MPs. Raising the stakes, the prime minister said the EU’s insistence amounted to a threat to the constitution of the UK: “We have been clear that we cannot agree to anything that threatens the integrity of our United Kingdom,” she added.

It wasn’t fine, but both sides have developed their thinking in the last round of tunnel negotiations. The British government asked for an all-UK customs union to replace the Northern Ireland specific backstop. The commission accepted that reference to such a customs union could be included, with a separate treaty laying out the details.

But they insisted that this would not obviate the need for a Northern Ireland specific text to offer cover should the negotiations over such a customs union fail later or if the UK chose to leave the arrangement.

EU officials do not rule out the possibility of the commission accepting that an all-UK customs union will replace the Northern Ireland-specific one, albeit with “deeper” clauses specific to the province. But that leap would require an admission from Downing Street that the customs union was effectively permanent, something unlikely to fly at home. It is a mess. But one French government official speaking for many in Brussels put it succinctly: “It is a British mess and they need to fix it.”