Effectively announcing his departure from front-line politics, Mr McGuinness, who has been severely ill in recent months, said he was resigning with “deep regret and reluctance”.
Appearing frail and in weakened voice, the 66-year-old insisted his health was not the reason for his resignation, claiming it was caused by the failure of First Minister Arlene Foster to stand aside to allow an investigation into so-called ‘Cash for Ash’ scandal. “My health is absolutely nothing to do with this whatsoever,” he said.
As they effectively share the office of First Minister, Ms Foster’s role will also cease.
The former IRA commander- turned-political leader failed to clarify if he would stand again for election, when asked by the media.
Mr McGuinness is said to be suffering from a rare heart condition and Belfast insiders believe his ill-health will not permit him to continue, ending the career of one of the North’s most controversial figures.
Political leaders in Dublin, Belfast, and London appealed for calm amid fears for the future of the Northern Assembly, with elections set to be called if Sinn Féin do not nominate an Assembly member to replace Mr McGuinness within seven days.
Mr McGuinness said he was stepping down from office because of the “arrogance” of Ms Foster’s handling of the crisis, which has gripped Stormont for several weeks.
Ms Foster insisted she would not temporarily step aside, pending an investigation into the renewable heat incentive (RHI) scheme, which could cost up to £490m (€563.5m), as demanded by Sinn Féin.
“The First Minister has refused to stand aside, without prejudice, pending a preliminary report from an investigation. That position is not credible or tenable,” said Mr McGuinness.
“We in Sinn Féin will not tolerate the arrogance of Arlene Foster and the DUP. I believe today is the right time to call a halt to the DUP’s arrogance. There will be no return to the status quo.”
Sinn Féin could re-nominate Mr McGuinness or a replacement to the deputy first minister role within the next seven days, but Mr McGuinness said that will not happen, saying fresh elections are the only way forward.
“We now need an election to allow the people to make their own judgment on these issues.”
Ms Foster said: “I am disappointed that Martin McGuinness has chosen to take the position he has today. 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 Féin’s actions are not principled, they are political,” she said.
“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.”
It will be a matter for the British government to call Assembly elections, with a late February or early March date likely. Prime Minister Theresa May has been updated on the resignation, said Downing Street.
Political leaders here expressed grave fears for the Assembly and the Government appealed for calm in Stormont.
Foreign Affairs Minister Charlie Flanagan said he “regretted” the circumstances which led to Mr McGuinness’s decision to resign his office.
“The Government is very mindful of the need to protect the integrity of the principles and institutions of the Good Friday Agreement,” he said.
“If, as appears likely, new elections to the Assembly will now be required, it behoves all parties to act responsibly in word and deed, so that the political institutions of the Agreement will not be damaged in the longer term.”
Mr Flanagan spoke to Mr McGuinness and to the British Secretary of State to Northern Ireland James Brokenshire.
“As a co-guarantor of the Good Friday Agreement, the Irish Government will continue to work with the British government and the political parties to advance political stability, reconciliation, and economic prosperity in Northern Ireland,” said Mr Flanagan.
Fianna Fáil leader Micheál Martin said it is with a sense of dismay that he has watched the so-called ‘Cash for Ash’ scandal unfold.
“The stage is now being set for a bitter election campaign 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,” said Mr Martin.