Sinn Féin has accused the DUP of "acting in bad faith" over commitments to deliver a promised Irish language act, raising fresh concerns about the future of the power-sharing executive at Stormont.
Arlene Foster is expected to tender her resignation as NI First Minister tomorrow, triggering a seven-day countdown for her replacement to be installed.
The DUP has said its choice for First Minister is Lagan Valley MLA Paul Givan, who at 39 years old would become the youngest person to hold the post.
But Sinn Féin has to accept that choice and agree to nominate a Deputy First Minister, who would be the current post holder Michelle O'Neill.
If it fails to do so, Northern Secretary Brandon Lewis is legally obliged to call an election "within a reasonable time".
A commitment to an Irish language act was part of the New Decade, New Approach (NDNA) deal brokered by Minister for Foreign Affairs Simon Coveney and the Northern Secretary at the time, Julian Smith.
It paved the way for the restoration of the Stormont Assembly after a three-year suspension.
New DUP leader Edwin Poots was one of the key architects, along with Conor Murphy, the Stormont finance minister and Sinn Féin's chief negotiator.
Mr Poots has said several times in recent weeks that he intends delivering on all the commitments, but there are concerns that internal party opposition will make that impossible.
There have been a number of engagements between the parties about the issue. Sinn Féin leader Mary Lou McDonald has met Mr Poots face-to-face three times, and Conor Murphy talked to him at length in the margins of the British Irish Council summit in Co Fermanagh on Friday.
Following those discussions, a senior Sinn Féin source today told RTÉ News the party's assessment is that "he is being disingenuous by saying publicly that he will honour commitments agreed in the NDNA".
They added: "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."
Speaking after the British Irish Council meeting in Fermanagh, Taoiseach Micheál Martin said it was imperative to protect the institutions of the Good Friday Agreement.
The Irish Government and Stormont Executive are due to take part in a North South Ministerial Council meeting in Co Armagh on Friday, but that cannot take place if Sinn Féin and the DUP fail to reach agreement.
In response, a DUP spokesman said: "No one would forgive Sinn Féin for playing fast and loose with people's lives in Northern Ireland.
"The DUP leadership stands resolute to enter government, respect power-sharing and get on with the job. It is up to others to follow.
"We remain committed to the New Decade New Approach agreement and want to see it implemented in all its parts."