Martin McGuinness has announced his resignation as Deputy First Minister of Northern Ireland in protest at the Democratic Unionist Party's handling of a botched renewable energy scheme.
The Sinn Fein veteran's move, which will come into effect at 5pm on Monday, is likely to lead to a snap Assembly election in the region.
Mr McGuinness announced his decision after his partner in government, DUP First Minister Arlene Foster, repeatedly refused to step down to facilitate a probe into the ill-fated Renewable Heat Incentive (RHI) - a scheme that has left Stormont facing a £490 million overspend.
Mr McGuinness said he was resigning with "deep regret and reluctance".
"The First Minister has refused to stand aside, without prejudice, pending a preliminary report from an investigation.
"That position is not credible or tenable."
He made clear that Sinn Fein would not replace him in the role.
As a consequence, the collapse of the institutions and an election are now inevitable.
"We now need an election to allow the people to make their own judgment on these issues democratically, at the ballot box," he said.
Mrs Foster presided over the ill-fated RHI while economy minister.
She has steadfastly refused to accede to Sinn Fein's demand for her to step aside to facilitate an inquiry into her actions.
The fate of the current DUP/Sinn Fein administration in Belfast now hangs on the pivotal issue of whether or not she will stand down.
The state-funded RHI was supposed to offer a proportion of the cost businesses had to pay to run eco-friendly boilers, but the subsidy tariffs were set too high and, without a cap, it ended up paying out significantly more than the price of fuel.
This enabled applicants to "burn to earn" - getting free heat and making a profit as they did so.
Claims of widespread abuse include a farmer allegedly set to pocket around £1 million in the next two decades for heating an empty shed.