British Prime Minister Theresa May is "dancing on head of pin".
Karen Bradley, Northern Ireland's Secretary of State, last month provided assurances that the British government will not repudiate the "backstop" commitment in Brexit negotiations, contradicting one of her party's main negotiators.
The Irish government said the Prime Minister and the Taoiseach had discussed a "review mechanism" in which the backstop could be ended by mutual consent.
The tough talk from Ireland came as shadow chancellor John McDonnell confirmed Labour would not support a temporary customs union with the EU. This would negate the need for checks at the Border.
The conversation followed a Daily Telegraph report that Brexit Secretary Dominic Raab has privately demanded the right to pull Britain out of the Irish backstop after just three months.
Irish Foreign Minister Simon Coveney spoke last month of "carnage" if Britain crashed out without a deal, though he said that would mostly be felt by the United Kingdom, with Ireland likely to benefit from solidarity from European Union states.
European Union diplomats said they doubted a deal could be completed in time for a tentatively scheduled summit on November 17-18, but that it could perhaps come a week later.
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Preparations for a final deal were far more advanced than previously disclosed, the report said.
The EU originally proposed keeping Northern Ireland alone under EU customs rules and other economic regulations in order to avoid disrupting the peace in the province by setting up barriers on the Irish border, the only EU-UK land frontier.
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A deal on the Irish border to break the Brexit deadlock is not close, the EU's chief negotiator has said. "The small print is that Ireland is f*****".
"The prime minister has been clear that we are making good progress on the future relationship and 95 percent of the withdrawal agreement is now settled and negotiations are ongoing".
"Still necessary to repeat this, it seems", she said.
The key idea would be that Great Britain and Northern Ireland would remain a single customs territory under World Trade Organization rules, linked in a customs union with Ireland and the rest of the EU, diplomats told Reuters.
Junior minister Damien English dismissed the suggestion that Ireland was "going to be shafted", insisting it was "not true".
The backstop plan enraged the DUP, which is propping up May's Conservative government at Westminster, and the PM has since said she could "not accept" any deal that would require a customs border between the North and Great Britain.
He said there was a "rapidly ticking clock" but that minds were now being focused with a view to concluding a deal and he welcomed that.
"He recalled the prior commitments made that the backstop must apply "unless and until" alternative arrangements are agreed".
Following a phone call with British Prime Minister Theresa May, Mr Varadkar said that Ireland will consider a "review mechanism" to the backstop issue which is causing an impasse in Brexit negotiations.