'Health has nothing to do with my decision,' says McGuinness
Martin McGuinness insisted that his health had "nothing to do" with his resignation as deputy first minister, as his decision sent Northern Ireland hurtling towards a divisive Assembly election.
Instead he blamed the DUP "arrogance" in the 'cash for ash' scandal for the shock move.
First Minister Arlene Foster hit back, accusing Sinn Féin of lacking principle.
Mr McGuinness's resignation came after Mrs Foster refused to step aside while the RHI scheme - which she established in 2012 while enterprise minister - is investigated. It has been claimed that flaws in the scheme left it open to abuse and the loss to the taxpayer in the North could be almost £500m (€575m). Ironically Mr McGuinness's move means that an inquiry into the RHI saga will be delayed amid the political chaos in the North.
Mr McGuinness, who is reported to be suffering from a rare heart condition, appeared frail as he answered questions on his decision to resign, and he has had to step back from his duties as in recent weeks for health reasons.
However, he insisted: "My health is absolutely nothing to do with this whatsoever". He said a decision on whether or not he will stand for election is a matter for a later date. In a statement, Mr McGuinness attacked the DUP, claiming that over 10 years the party, along with the British government, "have undermined the institutions and eroded public confidence".
"Sinn Féin will not tolerate the arrogance of Arlene Foster and the DUP," he added. "Sinn Féin wants equality and respect for all." Mrs Foster was quick to respond, accusing Mr McGuinness of depriving Northern Ireland of a government.
"I am disappointed that Martin McGuinness has chosen to take the position he has today," Mrs Foster said. "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 problems."
"It is clear that Sinn Fein's actions are not principled, they are political," she added.
"Let me make it clear, the DUP will always defend unionism and stand up for what is best for Northern Ireland, and it appears from the deputy first minister's resignation letter that is what annoys Sinn Féin the most," Mrs Foster said.
Foreign Affairs Minister Charlie Flanagan urged all parties in the North to protect the Good Friday Agreement as they faced fresh elections.
He said resolving the row over the RHI scheme was a matter for the Northern Executive and Assembly. But he urged the parties must act responsibly "so that the political institutions of the Agreement will not be damaged in the longer term."
Meanwhile, Fianna Fáil leader Micheál Martin hit out at both Sinn Féin and the DUP for failing to resolve the crisis over the RHI scheme. He said that instead "the stage is now being set for a bitter election that will not address any of the issues that led us to this point, and the future of the institutions is thrown into serious doubt".
Sinn Féin leader Gerry Adams responded to Mr McGuinness's decision by tweeting a picture of him signing his resignation letter. He wrote: "Ten years of valiant service in office of First & Deputy First Minister... GRMA [go raibh maith agat] chara 4 all Ur work."
Mr Adams later released a more substantial statement backing Mr McGuinness.
He said Mr McGuinness had "led from the front" in the Northern Ireland executive for ten years "defending the integrity of the political institutions and realising the potential of the Good Friday Agreement."
He argued that Mr McGuinness has always put the people and the political process first: "In spite of the provocation, disrespect and arrogance from the DUP, and the failures of the British government to fulfil its responsibilities over that time."