Former taoiseach (Irish prime minster) Bertie Ahern has accused Prime Minster Theresa May of “switching her language” over a physical border between Northern Ireland and the Republic after Brexit.
He told The Observer that “any kind of physical border” would be “bad for the peace process”.
Mrs May has said she wants a “seamless, frictionless border” after Brexit.
She also said that the ability of people to move freely across the border is “an essential part of daily life”.
In a statement on Sunday a UK government spokesperson re-iterated that both governments have made clear that “we want these seamless movements between our nations to continue.”
However, Mr Ahern, who played a key role in the Northern Ireland peace process, said she appeared to be changing her stance.
“(May) seems to be switching her language,” he said.
“She’s saying not that there’ll be no border, but that the border won’t be as difficult as to create problems.”
He added: “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 served as taoiseach between 1997 and 2008. He was one of the signatories of the Good Friday Agreement in 1998, but said that the post-Brexit border could put the agreement in jeopardy.
He said: “For the nationalist community in Northern Ireland, the Good Friday Agreement was about removing barriers.
“With so many other issues, there is a real concern… the only way (of) doing this will be 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, such as the one between Norway and Sweden.
“I haven’t found anyone who can tell me what technology can actually manage this,” he said.
Mrs May has previously said they she understood that the ability of people to move freely across the Irish border is “an essential part of daily life”.
She spoke after meeting with Taoiseach Enda Kenny at the end of January.
Mr Kenny also said that he has told the prime minster that “any manifestation of a hard border” would have “very negative consequences that (Mrs May) fully understands”.
A UK Government Spokesperson said: “Fourteen thousand people regularly commute across the border for work and study. Both the UK and Irish governments have made clear that we want these seamless movements between our nations to continue.”