Martin McGuinness, deputy first minister of Northern Ireland, has resigned over his power-sharing partner's handling of a controversial energy scheme.
Monday's surprise resignation is likely to bring down the region's government and lead to an election.
The development comes just weeks before the British government is due to trigger the process of leaving the EU, a decision that has divided Northern Ireland, which has Britain's only land border with the EU.
McGuinness's Irish nationalist Sinn Fein party had urged Arlene Foster, the pro-British first minister from the Democratic Unionist Party (DUP), to step aside over an energy scheme that could cost the province hundreds of millions of pounds.
"The first minister has refused to stand aside. Therefore, it is with deep regret and reluctance that I am tendering my resignation as deputy first minister," McGuinness said in a statement.
"Sinn Fein will not tolerate the arrogance of Arlene Foster and the DUP. We now need an election to allow the people to make their own judgement."
McGuinness, who has recently taken a break from some of his duties because of an undisclosed illness, told the British media that he would say at a later date whether or not he would be well enough to run as a candidate in any election.
McGuinness said Sinn Fein would not nominate anyone to fill the vacancy, meaning the power-sharing government will collapse, at the latest, once the seven days allowed for this elapse.
It would then be up to London's Northern Ireland secretary to propose a date for the election.
Sinn Fein says Foster failed to close down the green energy scheme, which aimed to encourage businesses to burn wood pellets rather than fossil fuels when it became clear that it was open to abuse.
The scheme gave businesses 1.60 pounds for every pound spent.
Foster says she closed it down as soon as the potential abuse was recognised.
Tension has been growing in recent months in the power-sharing government, created after a 1998 peace deal that ended three decades of violence between pro-British unionists and Irish nationalists who wanted to join the Republic of Ireland, and allowed border checkpoints to be dismantled.
Unlike Foster's DUP, Sinn Fein campaigned against Britain leaving the EU, and McGuinness has called for a referendum on Northern Ireland joining the Republic of Ireland to avoid an exit, which could see border checks reinstated.
Overall, 52 percent of the UK voted in favour of leaving the EU in June's referendum, but 56 percent of those voting in Northern Ireland backed remaining.
Source: News agencies