Assembly members at Stormont have rejected the British government's Brexit withdrawal deal.
The position was agreed without dissent following a near three-hour debate.
It was one of the first substantive acts of business since the assembly was restored earlier this month following the power-sharing deal.
Its two largest parties are the DUP and Sinn Féin, who oppose British Prime Minister Boris Johnson's Brexit deal for different reasons.
Northern Ireland has been unable to speak without the authority of elected ministers throughout most of the debate over Brexit due to the three-year suspension of the devolved institutions.
Sinn Féin Deputy First Minister Michelle O'Neill said: "We must work together with common cause to overcome the challenges that have been imposed on us by Brexit.
"This Assembly has not given its consent to the British government to legislate on our behalf."
The decision by assembly members will not affect the government's plan to leave the EU at the end of the month.
Northern Ireland voted in favour of Remain in the 2016 EU referendum, but there was a significant body of mainly-unionist opinion in favour of Leave.
The DUP is largely pro-Brexit, but opposes Mr Johnson's plan over fears it could impose customs checks on goods travelling between England, Scotland or Wales and Northern Ireland.
Nationalists and businesses are keen to ensure the Irish land border remains open and frictionless, and are mostly in favour of remaining within the EU bloc.
There have been fears leaving could encourage dissident republican violence or re-open the debate over Irish unity, since the Republic of Ireland remains a member state.
Speaking earlier, SDLP leader Colum Eastwood said the British government did not care about the people of Northern Ireland.
"It's important that this Assembly asserts its authority on behalf of the people of Northern Ireland," he said ahead of the debate.
"We have always said people here did not give consent for Brexit and it's important that this Assembly withholds consent for Brexit.
"We know this British government will ignore us but when they are ignoring us they are ignoring the people of Northern Ireland, people of Scotland and the representatives of the people of Wales.
"That shows you what kind of government that is sitting now in London. They do not care about people here, they don't care about people in Scotland, and they are determined to go on with the madness that is this Brexit.
"Next week we will be dragged out of the European Union against our will, against the will of people here and people in Scotland. That's why it's important that we're here, that's why it's important that we're rejecting it."
Mr Eastwood said the UK was facing a cliff-edge at the end of the year, insisting it would be impossible to strike a trade deal with the EU before then.
"So we are faced with chaos at the beginning of 2021," he said.