UUP's Nesbitt claims Sinn Fein 'putting self-interest above discovery of truth' over RHI
Sinn Fein has been accused of letting the DUP "off the hook" over the Renewable Heat Incentive (RHI) as Stormont was plunged into fresh turmoil.
The Assembly's main Opposition party the UUP also said the status quo of a DUP/Sinn Fein Executive was no longer sustainable.
However, Martin McGuinness's dramatic resignation as Deputy First Minister puts an inquiry into the botched RHI scheme on the back-burner.
The controversy that led to the senior Sinn Fein figure standing down yesterday will become a key issue in any election campaign.
UUP leader Mike Nesbitt said: "This is not the way to resolve the RHI scandal. Sinn Fein should have stayed, to hold the First Minister to account, to force a public inquiry and to vote on the much-needed cost controls on the scheme.
"Instead, they have prioritised self-interest, as always. 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."
As the clock ticked towards 5pm when Mr McGuinness's resignation became official, the UUP urged Justice Minister Claire Sugden to announce an inquiry into RHI before the deadline.
The only non-aligned minister in the Executive had pointed out she would need its approval before making such a decision.
SDLP leader Colum Eastwood echoed Mr McGuinness's insistence that the real reason behind the crisis was the "arrogance" of First Minister Arlene Foster.
"The public also knows that those behind the 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. The nationalist community also know that Sinn Fein have been the lead players in Government with the DUP for over a decade. Sinn Fein were jointly in control and are therefore jointly responsible."
Alliance leader Naomi Long said DUP arrogance had "recklessly endangered" the political institutions and the resignation "has brought to fruition a crisis which could have been averted with cool heads and calm leadership which put the interests of the people ahead of party political posturing".
Secretary of State James Brokenshire confirmed an election was now likely. "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."
But he added: "I would urge Northern Ireland's political leaders to take the necessary steps to work together to find a way forward."
Green Party leader Steven Agnew said: "There is an opportunity to pull back from the brink, but the Executive parties seem unwilling to put the electorate ahead of party interests at this point in time.
"I do not believe that the people of Northern Ireland want an election, coming so soon after the EU referendum and 2016 Assembly election."
TUV leader Jim Allister said it was now time to "move on" from devolution based on the concept of mandatory coalition. He said: "I've long said mandatory coalition would implode. Today it has.
"(There's) no point in sticking back together the fragments of failure."
Sinn Fein president Gerry Adams presented a stout defence of his party colleague.
"Martin McGuinness has led from the front in the Executive for the last 10 years, defending the integrity of the political institutions and realising the potential of the Good Friday Agreement," he said.
"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 - in contrast to the DUP."
Mr McGuinness's departure also provoked unease in Dublin. Irish Foreign Affairs Minister Charlie Flanagan said it now appeared new elections would be required, but the Assembly and Executive should not be "damaged in the longer term".
He added: "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."
Fianna Fail leader Micheal Martin warned: "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.
"Our fear is that party political interests have now overtaken the public interest."
Last night, Ian Paisley - whose father Lord Bannside served as First Minister alongside Mr McGuinness - claimed that Sinn Fein is struggling with "leadership issues".
The North Antrim MP said the inevitable Assembly elections were the making of their partners in government. He added that the unionist electorate will choose who leads the DUP and ultimately who will become First Minister - not Sinn Fein.
"If we are going to the country, then the people of Northern Ireland will choose who will be the largest party. And if the unionist people choose the DUP and our leader becomes the First Minister, then Sinn Fein have got to accept that if they like it or not," he said.
He added that the outgoing First Minister had agreed to a "full, judicial and independent" inquiry into the failed Renewable Heating Incentive.
"She said she has nothing to hide - why don't Sinn Fein get on with that and allow that to happen?
"If they find that there is some problem or issue with the First Minister, then at that point her position would become untenable.
"I think they know that she acted in good faith throughout all of this and they know they can't get her on that."
Asked if he still backs Mrs Foster, he replied: "Of course I do, I never had any doubt about it."