Sinn Fein's leader is expected to warn against progress to the next stage of Brexit negotiations without an answer to the Irish border question.
Mary Lou McDonald addresses the party's ard fheis (party conference) in Belfast on Saturday evening.
She will tell party members from around Ireland that if the architects of Brexit cannot agree what it looks like, that was their problem.
She will add: "It won't be our problem."
Sinn Fein is pressing for Northern Ireland to remain part of the EU Customs Union after the exit, and points to the majority in favour of Remain from the country during the referendum vote.
A deal has to be done on the border, one of the most vexed issues facing the negotiators in Brussels, by this autumn to meet the Brexit timetable before the British exit in less than a year's time.
If it is not done, a time-limited backstop option - which the EU interprets as Northern Ireland remaining in the Customs Union - would be introduced.
Theresa May's Cabinet in considering whether to support a customs partnership with the EU after Brexit, whereby Britain would effectively collect tariffs on behalf of the bloc, or a technological solution to border control known as "maximum facilitation".
Ms McDonald, who is addressing her first ard fheis as leader this evening, will say there cannot be progress to the next stage in Brexit talks "in the absence of an answer" to the border question.
"This is one test our government cannot shirk."
She will also restate that a powersharing ministerial Executive at Stormont was the right option.