A cross party trio of high-profile Remainers have accused the UK Government of ducking the Irish border question after holding talks in Dublin on the crunch Brexit sticking point.
Nick Clegg, Lord Heseltine and Lord Adonis were in the Irish capital on Wednesday at the start of a tour of European cities to gather evidence on the potential impact of the UK's exit from the European Union.
The Lib Dem, Conservative and Labour representatives met several senior politicians, including deputy premier Simon Coveney.
At an event hosted by Ireland's main opposition party - Fianna Fáil - the prominent Remain supporters spoke with one voice as they warned of dire consequences of Brexit.
All were scathing of the British Government's insistence that a hard border could be avoided in Ireland, even if the UK leaves the customs union and single market.
Former British deputy prime minister Clegg said: "We remain as perplexed as anyone else does about exactly how they are going to navigate this fundamental conundrum of creating a border but not wanting to police a border."
The ex-Lib Dem leader added: "Michael, Andrew and I come from different political parties, different political traditions but we are all united in a fervent belief that Brexit is not only being mis-handled at the moment but is also strategically a once-in-a-generation mistake of huge proportions for the United Kingdom while also weakening the European Union as well.
"We will go back to London with a strong message to the Government that they can't duck this issue any longer, they can't avoid confronting the consequence of their own contradictions any longer."
Lord Heseltine, himself a former Conservative deputy PM, joked that the UK Government's approach was like a Sherlock Holmes novel - "the case of the missing border".
He warned of the impact on the peace process if security checks returned at the 310 mile frontier between the North and Ireland.
"I am extremely worried that the issue of the border is simply kicked down the time zone," he said.
"Nobody has a serious answer to the questions, nobody has a solution that stands up to any sort of examination and therefore the easy thing for politicians is to have a few phrases and say 'yes, we'll sort that tomorrow', but the tomorrow is running out of time.
"Whatever politicians say, borders need to be policed and I know the only way you can police borders in modern society is with armed policemen. I think that would be tragedy beyond price for Ireland and the United Kingdom, yet the issue is being ducked, avoided, put off.
"This is unacceptable."
Lord Adonis insisted Brexit was not a "done deal" and there was "at least a 50/50 chance" of a referendum on the terms of the final withdrawal agreement.
"Brexit is not a done deal to an extent which I do not think is realised outside Britain," he said.
"There is a massive debate taking place in Britain, there is another year to go before we leave the European Union."
The Labour peer claimed a majority within the UK now favoured staying in the EU.
"The Achilles heel of Brexit is the border on Ireland," he said.
"Everyone says it would be disastrous to reimpose a border in Ireland but none of the people in favour of Brexit have any answers to the question on how you can have completely free trade and movement of people and goods between the Republic of Ireland and Northern Ireland and not have a border."
He added: "There is no border in the world that isn't policed."