Paul Givan has announced his resignation as first minister of Northern Ireland, as Democratic Unionist Party's protests against the Northern Ireland Protocol ramp up.
It means Deputy First Minister Michelle O'Neill, of Sinn Féin, also loses her position. Sinn Féin has called for an early assembly election as a result.
The protocol was agreed by the UK and EU to ensure free movement of trade across the Irish land border after Brexit.
However Unionists have been critical of the arrangements, saying they are damaging Northern Ireland's place in the UK.
"Today marks the end of what has been the privilege of my lifetime, to serve as the first minister of Northern Ireland," Mr Givan said on Thursday.
"Our institutions are being tested once again, and the delicate balance created by the Belfast and St Andrew's Agreements has been impacted by the agreement made by the United Kingdom and the European Union which created the Northern Ireland Protocol.
"The consent principle is a cornerstone of the Belfast Agreement and it is my earnest desire that all sections of the community will soon be able to give consent to the restoration of a fully functioning executive through a resolution to the issues that have regrettably brought us to this point."
In a press conference on Thursday, Sinn Féin president Mary Lou McDonald said the DUP's actions have consequences and called for an early election.
"We cannot stagger on in the months ahead without a functioning executive and Sinn Féin will not facilitate this," Ms McDonald said.
"In the absence of a functioning executive, an early election must be called and the people must have their say.
"We can do so much better than this chaotic theatre."
Northern Ireland Secretary Brandon Lewis said the decision by the DUP to withdraw from the executive was "extremely disappointing".
