Former taoiseach Bertie Ahern has warned that a post-Brexit hard border between Northern Ireland and the Republic could put the peace process in jeopardy.
In an interview with The Observer, Mr Ahern said Prime Minister Theresa may“seemed to be switching her language” on the issue.
In its Brexit white paper published last month, the UK government stated its aim to have “as seamless and frictionless a border as possible”.
But Mr Ahern, who played a pivotal role in bringing about the Good Friday Agreement, warned that any kind of physical border would be “bad for the peace process”.
He added: “She (May)is saying not that there’ll be no border, but that the border won’t be as difficult as to create problems.
“I worry far more about what’s going to happen with that. It will take away the calming effects [of an open border].
“Any attempt to try to start putting down border posts, or to man [it] in a physical sense as used to be the case, would be very hard to maintain, and would create a lot of bad feeling.”
Mr Ahern, who served as Irish prime minister from 1997 to 2008, expressed fears that the reaction of the unionist communities in the mid-1980s when the Republic was given an advisory role in the government of Northern Ireland could be repeated on the nationalist side if controls were reinstated.
He added: “It psychologically feeds badly into the nationalist communities.
“People have said that this could have the same impact on the nationalist community as the seismic shock of the 1985 Anglo-Irish agreement on unionists, and I agree with that.
“For the nationalist community in Northern Ireland, the Good Friday agreement was about removing barriers, integrating across the island, working democratically in the absence of violence and intimidation – and if you take that away, as the Brexit vote does, that has a destabilising effect.
“With so many other issues, there is a real concern … the only way [of] doing this will be a hard border.
“When people talk about hard borders, they’re talking about the borders of the past – but now any kind of border with checkpoints and security constitutes a hard border.”
Mr Ahern also told The Observer that he did not see how it would be possible to use technology to maintain an open border.
“I haven’t found anyone who can tell me what technology can actually manage this.”
During a recent meeting in Dublin, current taioseach Enda Kenny said he made it clear to Mrs May that “any manifestation of a hard border” would have “very negative consequences that [Mrs May] fully understands”.