The leader of Northern Ireland's Democratic Unionist Party, Edwin Poots, announced his resignation on Thursday just three weeks after formally taking power.
The resignation caused disarray in the region's largest party during crucial EU-British trade talks.
Hours earlier Mr Poots, who formally took control of the UK territory's largest pro-British party on May 27, was opposed by 24 of his party's 28 regional politicians on the nomination of a new first minister for the British province.
"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," Mr Poots said.
He said he would remain in his position until a successor is elected.
Mr Poots' resignation adds to political instability in Northern Ireland, which is the focus of a trade dispute between Britain and the EU, and has seen street violence fuelled by anger about restrictions on trade with the rest of the UK.
Objections by unionists and members of the Conservative Party to the trade restrictions have put pressure on British Prime Minister Boris Johnson to convince the EU to tear up the rules, a demand the bloc's leaders have repeatedly rejected.
Mr Poots' fellow DUP politicians had been angered by a move from the government in London to hasten the introduction of Irish language rights to try to convince Irish nationalists Sinn Fein to back the nomination of Mr Poots ally, Paul Givan, as First Minister.
Sinn Fein and the DUP had agreed to bring in the Irish language laws as part of a political agreement brokered by London and Dublin.
But many DUP members oppose introducing legislation on the language in the coming months.
Mr Givan was approved as leader of Northern Ireland's power-sharing executive, put in place by the 1998 peace deal that ended 30 years of sectarian violence, with Sinn Fein's consent.
After the vote, senior DUP politician Sammy Wilson said that "anyone who cannot bring their party along with them will find that they are not able to carry on anyhow."
Jeffrey Donaldson, who was narrowly defeated by Mr Poots in a leadership vote on May 14, would be a possible contender to replace him.