Martin McGuinness has resigned as Northern Ireland’s deputy first minister in protest against the handling of a botched energy scheme that could cost taxpayers £490m.
The Sinn Féin MLA’s resignation took effect from 17:00 GMT on Monday.
He cited the Democratic Unionist Party’s (DUP) conduct over the Renewable Heat Incentive (RHI) scheme scandal as the main reason.
His decision to quit is likely to lead to a snap assembly election.
- Live coverage: McGuinness quits
- Profile: A decade as deputy first minister
- QA: What is the RHI scheme?
- Analysis: When he goes, she goes
- BBC News NI reports on the RHI scheme
He said First Minister Arlene Foster has a “clear conflict of interest” in the RHI scandal and her position is “not credible or tenable”.
“Today is the right time to call a halt to the DUP’s arrogance,” he added.
Mr McGuinness said people should be allowed “to make their own judgement on these issues democratically at the ballot box”.
Prime Minister Theresa May has been “updated” on Mr McGuinness’ resignation, Downing Street has said.
The RHI scandal has engulfed politics in Northern Ireland over the past month, with DUP leader Mrs Foster facing repeated calls to resign – all of which she has refused.
She set up the RHI scheme in 2012 when she was enterprise minister in an attempt by the Northern Ireland Executive to encourage production of heat from renewable sources rather than fossil fuels.
But flaws in setting the scheme’s subsidy rate left it open to abuse as claimants could earn more cash the more fuel they burned, with the overspend estimated to be almost half-a-billion pounds.
In his resignation letter, Mr McGuinness said the DUP’s handling of the issue has been “completely out of step with a public mood”.
He said people were “rightly outraged at the squandering of public money”, and the DUP was refusing to accept demands for “robust action and accountability”.
“The refusal of Arlene Foster to recognise the public anger or to exhibit any humility in the context of the RHI scandal is indicative of a deep-seated arrogance, which is inflicting enormous damage on the executive, the assembly and the entire body politic,” he added.
Analysis: Mark Devenport, BBC News NI political editor
Under the joint protocols that govern Stormont’s power-sharing government, Martin McGuinness’ resignation means First Minister Arlene Foster also loses her office.
She may continue to exercise some functions, but her role will be very limited.
Sinn Féin has seven days to nominate a new deputy first minister, but the party is adamant it will not replace him.
An election could be called next week if Sinn Féin does not nominate a replacement as deputy first minister, and Mr McGuinness has said the party will not do that.
The veteran republican had been deputy first minister since the DUP and Sinn Féin made a historic agreement to share power in 2007.
But he said a number of issues in recent years had “raised all sorts of questions” about the behaviour of his party’s partners in government.
But DUP MP Sammy Wilson said the deputy first minister had stepped down “in a way to deepen this crisis”.
“It’s in everyone’s interests to try and pull back from the brink on this one and find a solution,” he added.
“We have worked very hard to try and achieve that including the offer of a full public inquiry.”
Speculation has surrounded Mr McGuinness’s health since he cancelled a planned trip to China with Mrs Foster last month on medical advice.
But he said: “My health has nothing to do with this whatsoever.
“I have been very deeply involved in all of the conversations in the course of recent weeks.
“I was here last week speaking to Arlene Foster, I’ve been doing my job as is appropriate for me to do so, so health has got absolutely nothing to do with it whatsoever.”