The UK’s take on the ‘backstop’ plan to avoid a ‘hard border’ in Ireland was in the wild for just 24 hours, before Michel Barnier eviscerated its central principle on Twitter. “To avoid any confusion”, the EU Commission’s chief negotiator tweeted, “our backstop cannot apply to the whole UK”.

At a stroke, the pretence disappeared that Brussels sought to adhere sincerely to the “joint report” on phase 1 of the negotiations. The repercussions could be particularly serious, if Theresa May’s deal-making with remainers in the House of Commons last night really has weakened Britain’s ability to drive a hard bargain.

Mr Barnier cannot possibly have forgotten the protracted wrangling that preceded agreement of the joint report, back in December 2017. The DUP vetoed an earlier draft of the document because it was insufficiently reassured that Northern Ireland would be treated the same as the rest of the UK, if the backstop applied.

During a week of leaks and speculation, a rumoured wording emerged that that would have provided for “alignment” across a limited number of areas, specified by the Belfast Agreement as matters for cross-border cooperation.

A broader, looser formulation appeared in the final text, promising that, in the absence of a trade deal or specific solutions for Ireland, “the United Kingdom will maintain full alignment with those rules of the Internal Market and the Customs Union which, now or in the future, support North-South cooperation, the all-island economy and the protection of the 1998 Agreement”.

Paragraph 49 was supplemented by a commitment, set out in paragraph 50, to avoid new regulatory barriers between Northern Ireland and Great Britain, unless the province’s devolved assembly decided specifically that unique arrangements for Northern Ireland were appropriate.

The government seemed to accept that it might have to align more closely with Brussels’ rules, as the price for securing a soft border without creating any new obstacles to trade within the UK. The DUP certainly believed it had ensured that Northern Ireland would leave the EU on the same terms as the rest of the country.

Almost immediately, there were signs that neither the harder-edged Brexiters, nor Barnier, accepted such a straightforward reading of the document. David Davis described it merely as a ‘statement of intent’, while the EU’s chief-negotiator said that, in his understanding, “alignment” was likely to apply only on the island of Ireland.

In March, the commission published its peculiar interpretation of the joint report backstop in its ‘Draft Withdrawal Agreement’. The promise that Britain would maintain “full alignment” with relevant rules had become a commitment to accept a “common regulatory area” on the island of Ireland, with Northern Ireland effectively staying inside the single market and the customs union, even while the rest of the UK left.

Although Theresa May rejected this notion, which would involve accepting an internal customs border at the Irish Sea, the government took until last week to set out its own understanding of the backstop, in a ‘technical note’ on a ‘temporary customs arrangement’. Because of this delay, and an apparent lack of enthusiasm for the contents of the ‘joint report’, for months the EU Commission has been allowed to present its provocative plan as the only solution on the table and gather support from remainers and nationalists in Northern Ireland, who see it as a way of edging the province closer to Dublin.

In a succinct internal slide-show, released publicly on Monday, the commission responded formally to the UK’s proposals. Its dismissive, confrontational tone formed a marked contrast to the government’s placatory language and assurances on the border.

The British ‘technical note’ included an extraordinary passage promising that the UK will “protect the Belfast Agreement in all its parts, including that there will be no hard border between Northern Ireland and Ireland, and no physical infrastructure or related checks and controls”.

The government continues to restrict its room for manoeuvre by insisting that the border will not be stiffened in any way, irrespective of the circumstances, or demands from the EU and the Republic of Ireland. The Irish foreign minister, Simon Coveney, indicated in November that his government will refuse to accept checks even if they take place well away from the frontier.

In addition, it now appears that the government accepts implicitly the inaccurate trope that a harder border is somehow incompatible with the Good Friday Agreement. Although similar claims have been made repeatedly, they’ve been rejected by the Supreme Court and rest mainly on hazy ideas about the ‘spirit’ or ‘context’ of the accord.

Whether the government intended to connect the agreement and the border so explicitly, or whether it was a result of saggy draftsmanship, it seems to concede an important point that could be used against it later, if the commission intends to maintain its hardline position. And many commentators think the EU has even less incentive to be reasonable now, if the government really has conceded a ‘meaningful vote’ in parliament on any final deal.

It’s one thing for Mr Barnier and his colleagues to reject the UK’s backstop as inadequate: the two sides are in a negotiation and the British document contained significant ambiguities around the arrangement’s likely duration and the extent to which Britain was agreeing to align with EU regulation. It’s quite another to insist that the only possible solution is a border in the Irish Sea and Northern Ireland’s full membership of the customs union and single market. On any ordinary reading, those demands obliterate paragraphs 49 and 50 of the ‘joint report’.

Perhaps it’s time for Theresa May and her government to temper their quiet diplomacy and remind the EU Commission more directly of its obligations under the December document.