Sinn Fein's Michelle O'Neill says Theresa May must call a border poll if reports of the Prime Minister's doubts Northern Ireland would remain in the UK after a referendum are accurate.
She insisted the Prime Minister had "no right to deny the people of Ireland the democratic entitlement to decide their own constitutional future".
Tuesday's Times reported Theresa May was not confident of certain victory in an Irish border poll amid Brexit wrangling after a confrontation with the MP Jacob Rees-Mogg.
“If these reports are accurate," Michelle O'Neill said. "Theresa May is conceding that the Good Friday Agreement threshold for triggering a unity poll has been met but that she isn’t prepared to allow the people of Ireland, north and south, to exercise their democratic right.
“Sinn Fein has raised the need for a unity referendum with the British Government on numerous occasions over recent years, particularly in the wake of the Brexit vote.
“On each occasion they have stated that they do not believe the threshold has been met but have repeatedly refused to clarify what criteria they use to reach this conclusion."
The Good Friday Agreement states there should be a border poll if the Secretary of State feels the majority would vote for unification.
MLA Michelle O'Neill continued: "There is nothing in that legislation that prevents a referendum from taking place at any stage. However, even if you accept the British Government’s interpretation of the threshold, then Theresa May’s comments are an admission that she believes the bar has now been reached.
“However, rather than act on that by honouring her government’s commitments and allowing people to exercise their democratic rights, she has also indicated that she intends to actively prevent that from happening.
“That is entirely unacceptable. It is an affront to the democratic rights of our people and the British Government must immediately clarify their position regarding their commitments to a unity referendum in line with their legislative and Good Friday Agreement commitments.”
The Times reports of a confrontation between the Prime Minister and the Brexiteer Jacob Rees-Mogg during briefings on Monday.
The briefings were - according to the paper's sources - aimed at "sending a tough signal" to hardline elements within her party she was not prepared to risk breaking up the Union.
Sources told the newspaper Mrs May and Mr Rees-Mogg clashed over plans for the Irish border.
The Times reports that Mr Rees-Mogg suggested the UK should keep the border between Northern Ireland and the Republic open with Mrs May countering that the EU could impose infrastructure to protect the integrity of the single market.
Sources said the backbench MP told Mrs May he had "no doubt" Northern Ireland would remain within the UK after any border poll.
Apparently Mrs May responded: "I would not be as confident as you. That’s not a risk I’m prepared to take. We cannot be confident on the politics of that situation, on how it plays out."
A Tory MP said Mrs May "slapped him down hard".
"She got him on facts.
"She was absolutely firm and passionate about the Irish position. I got a sense she realises what really matters," the MP told the Times.
Number 10 has been asked for a comment.