Stormont MLAs will vote before Christmas on whether to extend the post-Brexit trading arrangements that apply to Northern Ireland.
Northern Secretary Hiliary Benn has triggered the "democratic consent" process in a letter to Assembly speaker Edwin Poots.
The vote, which is provided for in the Windsor Framework agreement, only needs a simple majority of MLAs for it to pass.
The vote should be approved with Sinn Féin, the SDLP and Alliance MLAs all expected to vote in favour.
If that happens the arrangements will continue for another four years before there's another vote.
Under the Windsor Framework and the Northern Ireland Protocol, EU Single Market rules for goods are applied in Northern Ireland along with the EU customs code.
It means goods coming into Northern Ireland from Britain must provide paperwork to show they comply with relevant EU rules and are not intended for the wider single market.
Those rules have become known as the Irish Sea border.
Unionists have argued that such red tape both undermines the constitutional position and creates friction in the internal UK market.
Unionists have complained that the question of whether to continue the special arrangements should require cross community support, meaning that it would need a majority of both unionists and nationalists voting.
That would effectively hand a veto to unionism, all of whose representatives have pledged to oppose the move.
The First and Deputy First Ministers can bring the democratic consent motion to the assembly this month.
If that does not happen, any MLA may bring forward such a motion during the first week of December, with a vote no later than 17 December.
Mr Poots told MLAs in a letter he would prefer the debate and vote to take place at a sitting ahead of that date.
