Edwin Poots has resigned as DUP leader after an internal revolt over the restoration of the Stormont Assembly.
It comes just three weeks after he was confirmed as Arlene Foster's successor.
After attending a three-hour crisis meeting of party officers in the DUP's east Belfast HQ, Mr Poots said in a statement: "I have asked the Party Chairman to commence an electoral process within the party to allow for a new leader of the Democratic Unionist Party to be elected.
"The party has asked me to remain in post until my successor is elected.
"This has been a difficult period for the party and the country and I have conveyed to the chairman my determination to do everything I can to ensure both unionism and Northern Ireland is able to move forward to a stronger place."
The dramatic move came after the vast majority of DUP MLAs and MPs earlier voted against Mr Poots' decision to proceed with reconstituting the Stormont Executive, amid party anger at a UK government pledge to grant Sinn Féin a key concession on Irish language laws.
Mr Poots' announcement came after a tumultuous 24 hours in Northern Ireland politics - a period in which powersharing appeared to have dodged another crisis following the nomination of First and Deputy First Ministers in a special Assembly sitting, only for the administration to be plunged back into uncertainty hours later with Mr Poots' departure.
Serious question marks now hang over the future of newly appointed Stormont First Minister Paul Givan, who was nominated alongside Deputy First Minister Michelle O'Neill earlier today.
A planned meeting of ministers from the Northern Ireland Executive and Irish Government is not going to proceed as planned in Armagh tomorrow.
"Given political developments in Northern Ireland today, tomorrow's NSMC Plenary meeting has been postponed at the request of the Northern side," said a statement from the Irish Government.
'Three weeks exactly since Edwin Poots was ratified as DUP leader, he is no longer DUP leader, or certainly won't be soon' - Northern Editor @vincekearney reports from Belfast on a dramatic day of politics at Stormont | Read more: https://t.co/MCMxQ1hoQP pic.twitter.com/bi2WVqAhUa— RTÉ News (@rtenews) June 17, 2021
An electoral college made up of DUP MLAs and MPs will decide who will lead the party next. It remains to be seen whether it will be a contested race.
Sir Jeffrey Donaldson, who lost last month's leadership contest by 19 votes to 17, will be seen as a clear favourite by many.
In a statement this evening, a Sinn Féin spokesperson said that "whoever leads the DUP is a matter for that party".
The spokesperson said: "Sinn Féin has worked for weeks to bring stability to the Executive - our priority is our response to the Covid-19 pandemic, the continued successful rollout of the vaccination programme and rebuilding our economy to sustain jobs and livelihoods.
"We have monumental challenges ahead that will require unity of purpose and urgency. They include tackling the totally unacceptable hospital waiting lists that have left people crucified, in pain and without hope.
"That is our focus and should remain the focus of all ministers in the Executive."
Irish language sticking point
Earlier, elected DUP members strongly criticised a decision by the British government to promise to introduce Irish language legislation.
A majority of DUP Assembly members and MPs opposed nominating a First Minister.
They voted against Edwin Poots' decision to reconstitute the powersharing Executive with Sinn Féin in an internal meeting just minutes before the process for nominating Stormont's leaders began downstairs in the chamber of the Northern Ireland Assembly.
It is understood that Mr Poots and his choice of First Minister, Lagan Valley MLA Paul Givan, had left the room to head for the chamber just before the vote was taken.
One senior party source at the meeting, which happened in the members' dining room, described the atmosphere: "Dreadful. Utterly dreadful. Never experienced the like of it," said the source.
The DUP's ruling party officer team met in Belfast this evening to discuss today's developments and Mr Poots' position as leader.
Jeffrey Donaldson is the early front-runner to replace Edwin Poots as DUP leader, Northern Editor @vincekearney reports, while there is uncertainty about whether tomorrow's North-South Ministerial Council meeting will take place | Read more: https://t.co/MCMxQ1hoQP pic.twitter.com/uhV4aflPxs— RTÉ News (@rtenews) June 17, 2021
This morning, several DUP MPs and peers sent an urgent email to Mr Poots urging him to hold off nominating Mr Givan until he explained his decision to reassemble the Executive after Sinn Féin secured its key ask on Irish language laws.
A post-midnight announcement by the UK government committing to pass the stalled laws at Westminster in the autumn, if they are not moved at the Stormont Assembly in the interim, was enough to convince Sinn Féin to drop its threat not to nominate a deputy First Minister as joint head of the devolved Executive.
The development came after a night of intensive talks involving Northern Ireland Secretary Brandon Lewis and DUP and Sinn Féin delegations in Belfast.
Many DUP politicians had warned against a government intervention on a devolved issue and were furious that Mr Poots was still prepared to enter a new coalition on that basis.
The stand-off between the Executive's two main parties over the thorny language issue has been threatening the future of the fragile institutions in Belfast.
The issue came to a head this week as a result of the process required to reconstitute the Executive following the resignation of Ms Foster as First Minister.
The joint nature of the office Ms Foster shared with deputy First Minister Ms O'Neill meant her departure automatically triggered the removal of Ms O'Neill from her position - as one cannot hold post without the other.
While Mr Poots vowed to implement all outstanding aspects of the 2020 New Decade, New Approach (NDNA) deal that restored power-sharing, he declined to give Sinn Féin a specific assurance that he will move on the language element of the NDNA deal in the current Assembly mandate, a key demand of the republican party.
Amid the dispute, earlier this week Sinn Féin asked the UK government to step in and move the legislation at Westminster instead. DUP figures had warned Mr Lewis against such a step, characterising it as an overreach into devolution.
However, in the early hours of today, the Secretary of State announced that the government would table the language legislation at Westminster in October if Stormont had failed to do so by the end of September.
Mr Poots later voiced opposition to legislating on the issue at Westminster but said he would still proceed with nominating Mr Givan as First Minister.
The email subsequently sent to Mr Poots was signed by defeated leadership candidate Jeffrey Donaldson, party chairman Maurice Morrow, senior MPs Sammy Wilson, Gregory Campbell and Gavin Robinson, former deputy leader Nigel Dodds and a number of other senior members.
In total seven of the DUP's eight MPs signed the email, with Ian Paisley being the exception. The party's five peers also signed it.
Many of those who signed the email would have supported Mr Donaldson in his leadership bid, though some, like MP Paul Givan, supported Mr Poots' candidacy.
Accepting Mr Poots' nomination and taking the pledge of office during the special Assembly sitting, Mr Givan thanked Mr Poots for having "confidence in me".
He told the Assembly he shared the same "drive and determination" to serve the people of Northern Ireland as the party leaders before him.
He added: "There is much goodwill from the public for this place to work.
"We must recognise there is more in common than separates us. Northern Ireland is a special place."
After accepting her nomination during today's Assembly sitting, Ms O'Neill said: "We have monumental challenges ahead which require the same unity of purpose, the same urgency as we tackle the totally unacceptable hospital waiting lists which have left people crucified in pain and without hope.
"We must immediately set about addressing this issue together. We must mount a case to secure the funding from the British government to rebuild and transform our incredible public health service.
"Our people, and the heroic health service workers we are blessed with, deserve nothing less. Nothing less."
However, Traditional Unionist Voice MLA for North Antrim and former DUP member Jim Allister this evening said Edwin Poots had brought one of the worst days ever for politics to Northern Ireland and had given victory to ransom politics and he did not deserve to remain as leader of the DUP.
Speaking on RTÉ's Six One News, he said Edwin Poots had rolled over and done Sinn Féin's bidding and had lost his own party's support in the process.
Additional reporting - PA