EU diplomats have accused Theresa May of trying to delay resolution of the Irish border problem until after Brexit day by insisting upon Stormont having a final veto before any “backstop” solution can come into force.

Senior diplomats involved in the negotiations have reacted furiously to the details of a fresh UK proposal for avoiding a hard border on the island of Ireland, briefed to the Irish PM, Leo Varadkar, at last week’s Salzburg summit.

Under the solution, May will agree to Northern Ireland potentially staying, in effect, in the single market, as the rest of the UK exits after the transition period, should there be no other way to avoid a hard border at the time.

However, crucially, the UK is insisting that the Northern Ireland assembly, known as Stormont, would have to vote in support of this move before it came into force.

Stormont has not sat for 20 months due to the refusal of the DUP and Sinn Féin to work together. The assembly had a unionist majority from its establishment until the general election of 2017.

EU officials said the British government was seeking simply to push the issue into the future, leaving the backstop solution as an “empty shell”.

The backstop solution, which the EU says needs to be agreed in the withdrawal agreement if the UK is not to leave the bloc without a deal, is the largest stumbling block in the talks.

The UK has agreed there needs to be an insurance policy for avoiding a hard border should there not be a trade agreement or technological solution available by 31 December 2020.

However, the British government has until now rejected the EU’s proposal that Northern Ireland automatically stays in a customs union and under a large bulk of single market regulation as the rest of the UK leaves.

The British position on the single market, although not the customs union, has seemingly evolved, but only on the condition that there is a Stormont veto included.

One senior EU diplomat said there was no legal case for a sub-national body such as Stormont to have a role in the implementation of the withdrawal agreement once it had been ratified into a treaty.

The 1998 Good Friday agreement, which delivered peace to Northern Ireland, gave Westminster the competence to legislate to protect the terms of the peace accord, EU sources insisted.

A diplomat said: “This will not be acceptable to the Irish government nor Michel Barnier. This will be shot down straight away.”

May made a passing mention of her new terms in her bellicose statement the day after the Salzburg summit, where her Chequers plans for the future were torn up by the EU’s leaders.

May said she would soon publish her proposals, adding: “And it will be in line with the commitments we made back in December – including the commitment that no new regulatory barriers should be created between Northern Ireland and the rest of the UK unless the Northern Ireland executive and assembly agree.”

The row over Stormont has added to the tense nature of the talks on Northern Ireland, with EU officials further claiming that the Brexit secretary, Dominic Raab, had stopped engaging on the subject.

Beyond the issues of single market regulations, the bigger looming row is over how to avoid a customs border being drawn in the Irish Sea.

The EU’s proposal is that Northern Ireland stays in the customs union after Brexit, to ensure there is no need for infrastructure on the border with the Republic of Ireland.

May has repeatedly insisted that no British prime minister could sign up to a plan that it is claimed would constitutionally and economically split the UK in two.

Attempts to “de-dramatise” the issue of a customs border, through ensuring that the majority of checks took place away from ports and airports, has also been rejected by the UK.

A government spokesman declined to comment.

