Theresa May has been accused of putting Northern Ireland’s peace process in jeopardy by the Irish leader who helped to secure the Good Friday agreement.

In a sign of growing fears about May’s vision for Brexit, Bertie Ahern took aim at the prime minister over her recent white paper, in an interview with the Observer. Ahern, who served three terms as taoiseach between 1997 and 2008 and helped to deliver power-sharing in Belfast, said that the British government appeared to have resigned itself to the establishment of a border between the north and south once the UK leaves the EU in 2019, with potentially devastating results.

“[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. 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.”

In its Brexit white paper published last month, the government stated its aim to have “as seamless and frictionless a border as possible between Northern Ireland and Ireland”.

The secretary of state for exiting the EU, David Davis, has suggested that the arrangements between Norway and Sweden could be a model to copy, where CCTV cameras equipped for automatic number-plate recognition are in place. However, in an interview with the Guardian on Saturday, the European parliament’s Brexit negotiator Guy Verhofstadt appeared to scorn such a model, given that there would need to be customs checks and restrictions on the free movement of people.

Ahern said he, too, was unconvinced that the current technology could do the job. There are 200 crossing points on the border between the Republic and Northern Ireland, with 177,000 crossings by lorries a month, 208,000 by vans and 1.85m by cars.

“I haven’t found anyone who can tell me what technology can actually manage this,” Ahern said, adding that he feared the furious 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. “Any kind of physical border, in any shape, is bad for the peace process,” he said.

“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.”

Ahern’s comments were made as an EU document leaked to the Observer appeared to dash May’s hopes that the two states can come to a bilateral agreement. The British prime minister has repeatedly suggested that the 1923 Common Travel Area deal can be the basis for the future, although it was signed before either state joined the EU.

However, a memo from the European parliament’s legal affairs committee, which is helping shape the negotiating position of the European commission and the red lines of the European parliament, rebuffs that suggestion: “The [Good Friday] agreement makes it abundantly clear that the fact that both parts of Ireland and the UK are within the EU is a basis for the agreement. Moreover, the fact that Brexit could result in the reintroduction of border controls and controls on the free movement of persons between Ireland and Northern Ireland means this is a question for the EU, and not only Ireland the UK.”

The European Commission president Jean-Claude Juncker has said he fears Britain will divide the EU’s 27 remaining members by making different promises to each during its Brexit negotiations. “The other EU 27 don’t know it yet, but the Brits could promise country A this, country B that and country C something else and the end game is that there is not a united European front,” he told German radio.