The DUP has stated that a united Ireland is "nothing but a pipedream" in response to Sir Kenneth Bloomfield's comments at the Merriman Summer School in Co Clare on Thursday night that some form of unity was not unthinkable in principle.
The DUP and Ulster Unionist Party rejected Sir Kenneth's view that a united Ireland eventually could be achieved that would be mutually acceptable to unionists and nationalists.
However, Sinn Féin said the former head of the Northern Ireland civil service was reflecting a political reality while the SDLP said his comments helped to open up a much-needed debate.
DUP MLA Arlene Foster said unionists did not fear that a united Ireland was "just around the corner". There had never been less momentum towards a united Ireland, she said.
"Under DUP leadership, unionists are confident that our union with the United Kingdom is stronger than ever. Through ensuring that unionists hold the veto and are in control of their own destiny, we have safeguarded the union," she added.
"Sinn Féin may try to placate their supporters by arguing that they are on course for Irish unity by 2016 but it is widely accepted that a united Ireland is nothing but a pipedream," said the Fermanagh-South Tyrone MLA.
Ms Foster said she assumed Sir Kenneth was "speaking conceptually".
Danny Kennedy, deputy leader of the UUP, said that while Sir Kenneth was entitled to his personal view he was fundamentally wrong: "Unionists are not thinking in terms of even a managed or negotiated settlement involving a united Ireland.
"I certainly regard the Belfast Agreement and the product of the agreement, the St Andrews Agreement, as a political settlement and that that settlement will endure," he said.
"I don't believe Sir Kenneth's views are representative of the views within political or grassroots unionism. I think what he is saying is not on the agenda," added Mr Kennedy.
However, Sinn Féin MLA Willie Clarke said Sir Kenneth was reflecting wider opinion within Northern Ireland.
"No one should doubt that when an old-school unionist such as Ken Bloomfield can recognise the political dynamic of Irish reunification that he is reflecting the much broader political realities," he said.
"I believe that there is an unstoppable momentum towards Irish unity. This is reflected across all sections of our society and even privately many unionists recognise this reality. It is a question of how we manage this transition," said Mr Clarke.
In his speech Sir Kenneth warned against anyone believing that should 50.1 per cent of the Northern electorate vote for unity that "the outvoted 49.9 per cent would tramp into the new jurisdiction like a defeated army".
According to Mr Clarke, this highlighted how some unionists still had not grasped the principle of equality and could be interpreted as a "a threat that unionists would not accept the democratic wishes of a majority in any vote in favour of Irish unity".
SDLP deputy leader Dr Alasdair McDonnell said Sir Kenneth's speech meant that at last a proper debate on a united Ireland was beginning.
"The SDLP position is very clear - unity can only come through a process of dialogue and partnership. An agreed, united country is not a zero-sum concept or a winner-takes-all proposition," he said.
"All the guarantees, protections and equality provisions of the Good Friday agreement should apply in the context of a united Ireland. That is the framework in which we will build our shared future."
