A senior EU official has issued a blunt warning to the UK that it cannot solve the Irish border issue by making the so-called backstop guarantee on avoiding a hard border a UK-wide solution.
The senior official also flatly ruled out the new British proposal to make a backstop time-limited, and warned that time was running out for the UK to produce a legally operable text on the Irish protocol.
There was also a sharp warning that the Irish solution must involve full alignment of EU rules on goods, standards and regulations for Northern Ireland in order to avoid a hard border, and not just alignment on customs.
The dismissal follows intense speculation and debate over the British Prime Minister's latest attempt to forge a common position in her cabinet on Ireland and on the UK's future customs relationship with the EU.
Today's intervention by a senior EU official effectively scuppers the UK's latest gambit on the Irish border.
The senior official said: "What we need to have is the recognition that the backstop has to be Northern Ireland specific.
"We have to do away with the fantasy that there is an all-UK solution to that."
The latest approach from London is that the Irish border solution would be found by Britain temporarily applying the backstop - agreed in the Joint Report last December - to the UK as a whole.
The idea is that the backstop would be both UK-wide and temporary, with the UK deciding to align itself with the EU's common external tariff and its customs regulations for an extended period beyond the two year transition phase.
The idea would be that as technology improves, and the EU and UK strike a long term trade deal, there would be no need for a hard border.
However, this was flatly dismissed by the senior EU source, who said: "Let me make it very clear. The regulatory alignment option is not available on an all-UK basis because it would amount to selective participation in the single market, and we would need to understand the rules on the backstop for the customs dimension, which cannot be time limited."
Essentially the EU is arguing that for the UK to try to avail of the backstop on a UK-wide basis would amount to cherry picking.
The official made it clear that the British government was attempting to address the Irish backstop through the future trade relationship, when these were two separate things.
"We need to have a clearer understanding, or we need to have, ideally, agreement on the customs arrangement in the backstop which is to be looked at separately from the future EU-UK customs relationship," the official said.
"We need to have recognition that regulatory alignment for Northern Ireland is the key to avoiding a border. And we need to have made progress on the drafting of the protocol."
The Joint Report agreed between the EU and UK last December was fleshed out in a draft legal text in February.
The text will go on to form the overall Withdrawal Treaty which guides the UK out of the EU.
It contained a fifteen paragraph Irish protocol which, in the draft, foresees the territory of Northern Ireland remaining part of the EU's customs territory, and the full alignment of the rules of the single market for goods applying in Northern Ireland.
The text was immediately denounced by Theresa May and the DUP.
The official said that EU negotiators were open to an alternative text from London.
"We have always said we are open to alternative drafting from the UK. We only have one benchmark and that is, does the suggestion address the issue of the border while respecting the integrity of the single market and customs union?
"That is our benchmark. We will look at any drafting. So far we haven't got any. Progress at this stage seems illusive, and substantive progress even more so."
The official warned that progress was needed by June, and sharply hinted that the UK had to come forward with legal texts, rather than proposals that dealt with the EU-UK future relationship.
"On Ireland we need substantive progress in June because we are running out of time," the official said. "We need to draw up a legally settled solution in the Withdrawal Agreement by October. Here we need to work notably on the key issues of avoiding a border, [and] regulatory alignment which has to be Northern Ireland specific."
There was also a warning that the backstop had to be permanent.
"We pointed out [to British negotiators] that what they seem to be discussing will not address the issues linked to the border, and that of course, a backstop that would be strictly time-limited, would defeat the purpose of the backstop."
Meanwhile, speaking in Belfast today, the British Labour leader Jeremy Corbyn said his party will not support a Brexit deal that results in the re-imposition of a hard border on the island of Ireland.