Geoffrey Cox is facing a backlash in Brussels and Dublin after claiming the Irish backstop posed a risk to the human rights of people in Northern Ireland.

In the latest round of negotiations in Brussels, the attorney general told Michel Barnier, the EU’s chief negotiator, the arrangement could potentially breach the European convention on human rights (ECHR).

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It came amid increasing gloom in Westminster over the talks’ chances of success and as Cox hit back at EU complaints that his proposals to end the Brexit deadlock were too vague.

Cox said unless the backstop could be shown to be a temporary arrangement, it risked breaching protocol 1, article 3 of the convention, which protects the rights of people to vote in order to choose their legislature.

In what appeared to be a curve ball, Cox told the EU that Northern Irish citizens would be unrepresented in the EU’s decision-making institutions, including the European parliament, thereby diminishing their rights.

“The attorney general said there was a risk of violating the ECHR,” a senior EU source said. “He said a lot of surprising things this week.”

It is understood Cox argued the risk of breaching the ECHR put an onus on both sides to show the backstop was temporary, and should be pared back in scope as alternatives to its terms emerged.

Diplomats told of their astonishment at Cox’s claim, pointing out the UK had negotiated and agreed to the Irish backstop. “What is it they are trying to achieve with this?” one said. “They agreed on this arrangement in November and now it is a human rights risk?”

Politicians in Dublin expressed alarm over the apparent breakdown of talks.

“If you are resorting to using European human rights law, that means negotiations are not going well,” said Ireland’s shadow Brexit spokeswoman and Fianna Fáil TD, Lisa Chambers.

Chambers, who trained as a lawyer, said citing the European convention on human rights was a “very adversarial approach” more suited to the Old Bailey or the high court than negotiations about the future of the country.

“The problem is Geoffrey Cox is trying to solve a political problem with a legal solution. It makes you question the strategy of the government sending in a lawyer to negotiate when it is a political, not a legal, problem,” she added.

The EU’s deputy chief negotiator, Sabine Weyand, told ambassadors representing the 27 member states at a meeting on Wednesday that Cox was wrong, but did not elaborate. EU sources said, however, that there was nothing to Cox’s legal argument as Westminster is cited as the responsible legislature in the Good Friday Agreement, and it is a partner in negotiating the Brexit deal.

At attorney general’s questions in the Commons, Cox refused to be drawn on the details of the talks but said they would continue into the weekend.

“I am surprised to hear the comments that have emerged over the last 24 hours that the proposals are not clear. They are as clear as day and we are continuing to discuss them,” he said.

He refused to divulge the details of what he said were “detailed, coherent, careful proposals”, believed to centre on a new review mechanism for the backstop.

Weyand gave what sources described as a “gloomy” briefing to ambassadors on Wednesday night about Cox’s visit, as well as that of the Brexit secretary, Stephen Barclay, to Brussels, where they were seeking reassurances about the temporary nature of the backstop ahead of a Commons vote on Tuesday on the withdrawal agreement.

Cox has been pushing for the arbitration panel to be able to decide whether the EU has made reasonable efforts to negotiate alternatives to the backstop, opening up the opportunity for the UK to exit the arrangement in part or whole.

Weyand said that this could lead to what she described as a “mini-backstop” in which Northern Ireland remained in elements of the regulations cited in the Irish protocol of the withdrawal agreement.

Diplomats said that the ability of the arbitration panel to judge this, and without recourse to the European court of justice unless EU law was at stake, would “go well beyond what is in the withdrawal agreement and would undermine the EU’s legal order”.

The EU is offering to convert a series of pledges, made in a letter in December by the European commission president, Jean-Claude Juncker, and his European council counterpart, Donald Tusk, into legally binding commitments in a so-called joint interpretative instrument.

Those include stating the EU’s “firm determination” to have an alternative to the backstop ready before the end of 2020, to avoid it being triggered. Should the customs union it envisages come into force, both sides would set the “objective of making this period as short as possible”.

Such commitments could be used in arbitration by the UK should there be any doubt about the “good faith” of the EU negotiators in subsequent talks. But Cox is said to have been going beyond what Barnierwas able toaccept, EU sources said.

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Meanwhile France’s Europe minister, Nathalie Loiseau, told the Guardian the EU was “still waiting for a proposal from London” to solve the impasse. “At this stage we know what the UK does not want, and that’s a first phase, but it’s not necessarily enough,” she said.

Loiseau, who was in the UK on Thursday, said Brussels was still awaiting a “sustainable proposal” on the backstop.

Asked by the Conservative Brexiter Bill Cash if more talks were expected, Cox said: “Of course these discussions are running, they’re going to be resuming very shortly, they’re going to be continuing, almost certainly, through the weekend, and we will endeavour to give the house as early notice as well can if and when we have something to report.”

Cox said it was still his intention to secure “legally-binding changes to the backstop which ensure that it cannot be indefinite”. However, he faced a series of hostile questions from Brexit-minded Conservative MPs concerned at a lack of progress after talks that even Downing Street described as “difficult”.

Sunday night is the final deadline for any changes, as the government needs to publish and print copies of deal documents on Monday, and publish the motion MPs will then vote on.