By David Young and Michael McHugh, Press Association
Power-sharing in Northern Ireland has been engulfed in a crisis fuelled by a green energy scandal after Martin McGuinness quit and pulled down Stormont's ruling executive.
The Sinn Fein Deputy First Minister resigned in protest at the Democratic Unionists' handling of a botched scheme that has left the administration in Belfast facing a £490 million bill.
His decision to walk away after 10 years sharing power with the DUP came as First Minister Arlene Foster refused to stand aside to facilitate a probe into the ill-fated Renewable Heat Incentive (RHI) - the so-called "cash for ash" furore.
The UK Government is set to call a snap Assembly election.
DUP leader Mrs. Foster oversaw the doomed RHI during her time as economy minister. She has rejected Sinn Fein's demands to step down temporarily pending the outcome of a preliminary investigation.
Under the structures of the peace process-forged institutions, neither Stormont's First Minister nor Deputy First Minister can remain in post without the other, so Mr. McGuinness's resignation spelled the end of Mrs Foster's current tenure in the job.
"We in Sinn Fein will not tolerate the arrogance of Arlene Foster and the DUP," he said. "I believe today is the right time to call a halt to the DUP's arrogance."
Mr. McGuinness denied his health problems, for which he is undergoing intensive treatment, had influenced his move.
In response, the outgoing First Minister accused Mr. McGuinness of putting politics before principle.
Mrs. Foster said she was "disappointed".
"His actions have meant that, at precisely the time we need our Government to be active, we will have no government and no way to resolve the RHI.
"It is clear that Sinn Fein's actions are not principled, they are political."
Mr. McGuinness made clear that Sinn Fein would not replace him in the role.
While politicians are set to face the electorate, a poll is unlikely to resolve the crisis if the RHI issue is not dealt with before an executive is re-formed.
Sinn Fein has warned there will be no return to the "status quo".
Northern Ireland Secretary James Brokenshire said: "Unless Sinn Fein nominates a replacement to the position of Deputy First Minister within the next seven days, it is incumbent upon me to call an Assembly election within a reasonable period."
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.
While the DUP and Sinn Fein were in agreement on the terms of a potential investigation into RHI, the sticking point was the position of Mrs. Foster when the probe got under way.
Steps by the Executive to cut the costs of the overspend will not be implemented in the short term.
Mr. McGuinness cited other disputes with the DUP, including over the Irish language and stalled mechanisms to deal with the legacy of the Troubles, in explaining his move.
The DUP claimed RHI was not the motivation behind Sinn Fein's strategy, insisting they were exploiting the crisis to pursue a broader republican agenda.
A spokeswoman for Theresa May said: "The Prime Minister has been kept updated on the resignation of Martin McGuinness.
"The Northern Ireland Secretary James Brokenshire is in contact with people there, encouraging all parties to continue the dialogue."