By Michael McHugh, Press Association
Sinn Fein have let the Democratic Unionist Party (DUP) off the hook, the leader of the Opposition at Stormont said.
A public inquiry should be held into the failed Renewable Heat Incentive (RHI) scheme so all the facts are exposed and cost controls are put in place, Ulster Unionist chief Mike Nesbitt said.
Martin McGuinness resigned as Deputy First Minister in protest at the DUP's handling of a botched renewable energy scheme. The UK Government has urged Northern Ireland's political leaders to work together and find a way forward.
Mr. Nesbitt said "This is a sad day for Northern Ireland."
He accused Sinn Fein of prioritising self-interest, as always.
He said: "This is Sinn Fein letting the DUP off the hook. The public mood clearly indicates they want the facts of the RHI debacle exposed.
"To move straight to an election without this taking place is farcical.
"They had a choice between the integrity of the institutions and electoral advantage and they appear to have chosen the latter."
He said it was proof of the coalition partners' inability to govern.
"This scandal-prone Executive has had 10 years to get it right," he said. "That's longer than most get in a mature democracy. They promised a Fresh Start, but delivered a fresh crisis of momentous scale."
Northern Ireland Secretary James Brokenshire said the Government would do all it could to help the parties find a resolution.
Unless Sinn Fein nominates a replacement to the position of deputy First Minister within the next seven days, Mr Brokenshire said he will have to call an Assembly election within a reasonable period.
He said: "I would urge Northern Ireland's political leaders to take the necessary steps to work together to find a way forward and I will work with all parties and the Irish government to this end."
Photo: Niall Carson/PA Wire.
Social Democratic and Labour Party (SDLP) leader Colum Eastwood said First Minister Arlene Foster's "arrogance" had caused a potential election.
He claimed those behind "cash for ash" scandal can now enjoy a two-month break from any effective public inquiry or police investigation.
He said: "It ensures no immediate consequences for Arlene Foster, no emergency legislation to cap costs and no investigation into potential corruption. Each and every day, this scandal is costing taxpayers £85,000.
"If the Executive is incapable of opening itself up to accountability, I would urge the electorate to enforce their accountability at the ballot box."
Traditional Unionist Voice (TUV) leader Jim Allister said there should be a voluntary, rather than forced, coalition.
He said: "People in Northern Ireland deserve good government, in contrast to what they have they deserve government.
"They have not got it. There is no point in simply trying to glue back together that which will never work.
"So let's step out of this shambles, into the light of something that can and will work."
Sinn Fein president Gerry Adams said Mr. McGuinness had led from the front in the Executive for the past decade, defending the integrity of the political institutions and realising the potential of the Good Friday Agreement.
He said: "He has worked with successive DUP leaders and is recognised throughout the north, across Ireland and across the world for his leadership and peacemaking.
"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, Martin McGuinness has always put the people and the political process first.
"This is in contrast to the DUP who have been acting to undermine equality and partnership.
"The money squandered in the RHI project belonged to unionists as well as other taxpayers. It is money which should have been used to end poverty and disadvantage or to build public services. No minister responsible for such bad governance in any other administration would be still in office.
"Today, once again, Martin McGuinness has taken decisive action to bring the RHI scandal, and the DUP's attitude to the political process, to a head."
Alliance leader Naomi Long said the DUP has "recklessly endangered" the political institutions.
She said the public clearly want the First Minister to step aside without prejudice for the duration of an inquiry into the RHI scandal, but her "inability to swallow her pride" and her "belligerent attitude" has placed her party on a collision course with Sinn Fein, which with the resignation of Mr. McGuinness has now raised the stakes and potentially set the course for an election.
"This is a head-on collision that has been building for some time," she added.
"The DUP have continued to ignore the voices who reasonably asked the First Minister to step aside voluntarily and without prejudice to allow a public inquiry into RHI, to get to the truth of this matter."