Brexit posters are pictured at the Border Crossing at Muff in Co Donegal near Lough Foyle on the border with Northern Ireland and Donegal in the Republic of Ireland | Paul Faith/AFP via Getty Images Invisible Irish border won’t work, says former minister ‘I think it’s time there is an outbreak of common sense in London,’ says Dick Roche, Ireland’s former Europe minister.

A former Irish minister for Europe supported new Prime Minister Leo Varadkar's tough stance on the Irish border in Brexit negotiations and called for more clarity from the British government.

Speaking on BBC Radio 4's Today program on Wednesday, Dick Roche, who twice served as Ireland's minister for European affairs under Fianna Fáil governments, said a so-called invisible border between the Republic of Ireland and Northern Ireland simply would not work.

"It's wishful thinking; the reality of it is the border between Northern Ireland and the Republic is an extraordinarily porous border and always has been," said Roche. "We saw that at the height of the Troubles, when there was a very, very large British Army presence along there. It still didn't stop illegal activities along the border and we know that that's a reality."

Varadkar has repeatedly said he does not want to see an economic border between Northern Ireland and the Republic, adding that he does not believe an invisible border — using the likes of camera recognition technology and preregistered cargo — would be effective. Although Varadkar is from a different party, Fine Gael, Roche stood by the PM.

"I think he's absolutely correct in being forthright in saying that it won't work," said Roche. "There is a solution if everybody sits down and thinks in terms of an all-island solution where we can continue to have free travel for persons and with very little disruption, we could continue to have free movements of goods."

Roche also called on the British government to provide more clarity on its Brexit position.

"I think it's time there is an outbreak of common sense in London," said Roche. "The unionist population and the unionist politicians in Northern Ireland have to be brought closer into the discussions. But we still don't know what the British government's position is actually going to be. And this is so frustrating. I mean it's boiling frustration under the surface here in the Republic because this is so critical for the wellbeing of the entire island."

Roche added that Varadkar should stop further progress in Brexit negotiations if U.K. officials cannot present a working plan for the Irish border.

"I think sometimes you have to speak on strong terms to even your best friends. We've a great relationship with the U.K. ... We have a good relationship north and south of the border since the agreements. And it would be absolutely reprehensible if politicians don't wake up to the realities that without some creative thinking, they're not going to solve this particular problem."