Sinn Féin has accused DUP leader Edwin Poots of acting in "bad faith", and said that the nominations of a first minister and deputy first minister at Stormont this week have to be accompanied by the progression of Irish language legislation.
Sinn Féin's conclusion that they "do not believe" Mr Poots will deliver on the Irish Language Act could spark a political crisis at Stormont.
When Arlene Foster formally resigns as First Minister on Monday, deputy First Minister Michelle O'Neill is automatically removed from her post as well - as the joint office can only function if both positions are filled.
Both parties will then need to re-nominate their respective first and deputy first ministers within seven days.
If one of the parties declines to re-nominate, or if either nomination fails to gain sufficient Assembly support, then a functioning Executive cannot be formed and a snap election will become likely.
It is understood Sinn Féin had been seeking firm guarantees from Mr Poots on issues such as Irish language as a prerequisite for its engagement in the nomination process.
A senior Sinn Féin source said: "Sinn Féin has scoped out Edwin Poots and the DUP, having had a series of engagements where we've talked and listened up until yesterday.
"Our assessment is that he is being disingenuous by saying publicly that he will honour commitments agreed in NDNA (New Decade, New Approach).
"We believe they are acting in bad faith. We do not believe they will deliver on the Irish Language Act.
"Our position is that the nomination for first minister and deputy first minister has to be accompanied by legislation on the Irish language."
Mr Poots has previously pledged to implement Irish language legislation at Stormont as quickly as possible.
The new DUP leader said last week he wished to "expedite" the rollout of all outstanding aspects of the New Decade, New Approach (NDNA) deal that re-established power-sharing in 2020.
The cultural elements of NDNA, include protections for Irish and Ulster Scots, would have to be delivered in the form of amendments to the 1998 Northern Ireland Act.
Sinn Féin wants those legislative changes passed before the end of the current Assembly mandate next spring.
It is understood the party wants to see progression of the amendments introduced in tandem with the process of nominations for first and deputy first minister.
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