Leo Varadkar dismissed Ms May’s claim that negotiations on the future land border are “almost there” as “wishful thinking”, at a breakfast meeting.
Instead, he told the UK prime minister that she must set out detailed proposals that can form part of the conclusions of the crunch December EU summit.
Without that reassurance, the EU would block any attempt to move the negotiations onto future trade and a transitional period to cushion Brexit – the Holy Grail for the UK.
“They want us to take a leap in the dark and we are not prepared to do that,” one Irish source told The Independent.
“The British want to give the impression that we are all on the same page, that it is just a question of finding a form of words, but that is certainly not the case.
“We need an explicit commitment, confidence about the impact on the island of Ireland, before the talks can progress to phase two.”
It is understand that Ireland would not exercise its veto, but has no doubt that all 27 EU countries would unite in drawing a red line to frustrate the UK.
The stance, following talks between the two leaders at a summit in Sweden, is a stark reminder that the so-called “divorce bill” is not the only remaining obstacle to breaking the deadlock in the negotiations.
However, No 10 insisted there had been “constructive discussions on Brexit” and that both leaders anticipated “further progress” before the EU council.
“On Northern Ireland, the PM was clear that the Belfast agreement must be at the heart of our approach and that Northern Ireland’s unique circumstances demand specific solutions,” a spokesman said.
“The PM said it was important to protect progress made in Northern Ireland over recent years. Both leaders agreed to work together to find solutions which ensure there is no return to the borders of the past.”
British negotiators had, until recently, hoped that the vexed issue of the Irish border could be “parked” until trade talks begin, because they are so closely linked.
However, a leaked European Commission earlier this month showed that Dublin has Brussels staunch support in ensuring the controversy remains a priority.
It made clear that, in order to preserve the Good Friday Agreement, the Brexit divorce deal must respect “the integrity of the internal market and the customs union”, with Ireland remaining a member of both.
That meant the UK, to avoid a hard border with Northern Ireland, must also remain part of the customs union – something London has categorically ruled out, at least long term.
Previously, Brussels has dismissed the UK’s proposals to use untested new technology to police a light-touch Irish border as “magical thinking”.
Dublin is said to be demanding that the UK preserve about 100 EU regulations, many covering customs and agriculture, to ensure an open trade border with Northern Ireland.
But that would mean London either accepting the rules for the whole of Britain or granting special status to Northern Ireland, weakening the integrity of the UK – which ministers have rejected.
Meanwhile, in Dublin, Foreign Secretary Boris Johnson told his Irish counterpart, that only a progression to trade talks could solve the border issue.
But Simon Coveney said Britain would have to bend if it was to stand by its promise of no return of “physical infrastructure”.
“We simply don't see how we can avoid border infrastructure, whether it's on the border or somewhere else on the island, if we have regulatory divergence in Northern Ireland versus the rest of the island,” he said.
“When you have a different rule book applied to trade and business, well then, you are starting to go down the road of having to have checks and inspections.”
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