Ulster Unionist leader Sir Reg Empey has warned the Government against allowing MPs from Northern Ireland to take part in debates in the Dáil and Seanad.
It follows commitments by Taoiseach Bertie Ahern to Sinn Féin to examine the issue.
Sir Reg Empey said the proposal was outside the terms of the Belfast Agreement, and was a breach of the principle that the consent of the majority of people of Northern Ireland would be required before any change to the North's constitutional status.
"I believe this move is very dangerous because it would effectively be setting up an embryonic all-Ireland parliament," the East Belfast Assembly member said.
"When the idea was first mooted two years ago the UUP opposed it. We told the two governments then, and have repeatedly since, that if it is pursued by Dublin we will no longer be obligated to our support for North-South institutions."
On Sunday, Minister for Justice Michael McDowell appeared to rule out giving the North's MPs an automatic right to address the Dáil, claiming it would not be in line with the Belfast Agreement.
Mr McDowell said although the Taoiseach had promised to raise the matter with party leaders in the Oireachtas, it would require all-party agreement. "I don't believe there's all-party agreement. Certainly the Progressive Democrats don't agree that any person should be entitled as of right to address the Dáil because they're elected to Westminster."
There are other options that are expected to be examined, including allowing MPs to address specific Oireachtas committees dealing with Northern Ireland affairs and to enable a number of senators to be directly elected by the Northern Ireland electorate.
Yesterday Sinn Féin vice-president Pat Doherty confirmed it was a demand his party's negotiators had pressed for.
"The Irish Government have given commitments to facilitate this as an expression of the right of Northern nationalists, of Irish citizens, to participation in the political life of the nation.
"I welcome any move by the Irish Government to give effect to these commitments."