Tánaiste Simon Coveney has accused Boris Johnson of looking to "rewrite the rules" and dismissed suggested alternatives to the backstop as the Government continues to ramp-up its no-deal Brexit preparations.
Mr Coveney struck a notably pessimistic tone on a trip to the Czech Republic yesterday and sharply criticised Mr Johnson, who has demanded the backstop be removed from the withdrawal deal.
The Irish Government's no-deal preparations are intensifying further with 64 days left until the UK is due to leave the EU.
Finance Minister Paschal Donohoe has warned the "risk of a no-deal Brexit is growing".
Mr Coveney was highly critical of Mr Johnson, who became UK prime minister last month, on a visit to Prague.
"Now we have a new prime minister who is essentially looking to rewrite the rules as regards that particular solution [the backstop] and that is threatening a no deal, having worked so hard to get a deal in place," he said.
He said that while the Government was open to new alternatives that fulfil the same functions as the existing Withdrawal Agreement's backstop - the guarantee of no hard Border in Ireland in the event that the UK and the EU can't reach a future trade deal - none that had been put forward so far would be sufficient.
"I think we need to be honest here that the alternative arrangements that have been discussed to date do not do the same job as the backstop - not even close," he said.
"So let's not pretend that solutions exist when they might not, and so there needs to be a deal done on the basis of honesty in terms of there issue and the complexity of what we're facing."
He said Ireland would lose a powerful ally in the EU when Britain left.
"It will certainly be a different union for Ireland, it will be a weaker union," he said.
Mr Coveney was speaking after Mr Donohoe warned in Dublin there is a growing chance of a no-deal Brexit.
"As we look at the unfolding situation in the UK, the risk of a no-deal Brexit is growing," he said.
Mr Donohoe said the Government would be making a judgment on the most likely Brexit scenario next month.
He said he expected there would be a review of where Brexit stands at next week's Cabinet meeting - the first after the summer break.
Earlier, Culture Minister Josepha Madigan struck a more hopeful tone and even claimed Mr Johnson could still back the Brexit withdrawal deal at the very last minute.
She said there was "a lot of posturing" going on in the current Brexit deadlock and said that despite the ongoing stalemate, there could be a deal at the very last minute, with Mr Johnson reversing his position.
"I do understand negotiations, I do understand that there is posturing, I do understand that people sometimes might say one thing in private and another thing in public," Ms Madigan said at an event in Dublin yesterday morning.
She said it "remains to be seen" whether Mr Johnson would follow through on his pledge to leave without a deal if necessary and suggested he could yet back the deal.
"Whether what he's saying is what he'll actually follow through with remains to be seen," she said.
"He did vote for the Withdrawal Agreement in the past, it may well be at the last minute that happens.
"Again, we'll just have to take each day as it comes and hope that sense will prevail."
Ms Madigan said it was possible the Brexit deadlock would not be resolved until the very last minute, and potentially as late as October 29 or 30.
She suggested that, despite the uncertainty this could create over the next few weeks, it would not be a bad outcome in the end.
"It wouldn't surprise me if we were, and if it were at that stage and we reached a deal, isn't that OK as well? We have (until) October 31, at the end of the day," she said.
Fianna Fáil Brexit spokesperson Lisa Chambers dismissed the suggestion of a U-turn by the prime minister.
She added: "Even if Mr Johnson did a major U-turn and backed the deal in its current form, I think the arithmetic of the Commons won't facilitate its passage.
"He can't bring back the same cooked dinner that he has continually and publicly turned his nose up at."
