The Conservative Party has warned against the creation of "a 32-county Dáil in shadow form", and said proposals to allow Northern Ireland politicians speaking rights in the Oireachtas would undermine the principle of consent.
Sinn Féin's long-standing demand that Northern Ireland MPs, MEPs and Assembly Members should have association rights in the Dáil has moved back up the political agenda on foot of last week's IRA statement formally ending its armed campaign.
However Conservative shadow Northern Ireland secretary David Lidington says any such offer to people elected to serve in UK legislatures "will look to many in Northern Ireland and Great Britain like an attempt to bypass the principle of consent and establish a 32-county Dáil in 'shadow' form".
In a letter to Irish Ambassador Daithí Ó Ceallaigh in London, Mr Lidington says: "Clearly, the Irish Republic, like any other sovereign state, is entitled to decide who should sit and speak in its legislature, but I am troubled by the way in which this proposal appears to challenge the principle of consent which is such a fundamental part of the Belfast Agreement."
Mr Lidington continues: "The agreement binds both the United Kingdom and the Irish Republic to accept the current constitutional status of Northern Ireland for as long as the people of Northern Ireland so wish.
"If the principle of consent means anything, it must mean that the people of Northern Ireland elect representatives to serve in Westminster and Belfast unless and until they decide democratically to join the Irish Republic.
"Now, to offer rights of representation in Irish institutions to people elected within the jurisdiction of the United Kingdom to serve as representatives in United Kingdom legislatures will look to many in Northern Ireland and Great Britain like an attempt to bypass the principle of consent and establish a 32-county Dáil in 'shadow' form."
Asking for additional detail of what the Irish Government proposes, Mr Lidington has also asked Mr Ó Ceallaigh to brief him "on how your Government believes this move to be compatible with the agreement".
A Conservative spokesman told The Irish Times that Mr Lidington's letter underlined "very real concern about this proposal".
And while again stressing it was ultimately a matter for Irish decision, the spokesman said "it is very, very unhelpful in terms of fostering genuinely good relations within Northern Ireland".