Doug Beattie has quit as leader of the Ulster Unionist Party, citing "irreconcilable differences" with party officers for the shock move.
Mr Beattie issued a statement confirming he was standing down after a morning of intense speculation over his future amid apparent internal tensions with other senior party figures.
The Upper Bann Assembly member, who is a British Army veteran, was elected UUP leader in 2021.
From the liberal wing of unionism, the Military Cross recipient took on the role pledging a progressive agenda and a move toward the middle ground.
His resignation comes after what was a relatively positive general election result for the UUP, with the party securing a Westminster seat for the first time since 2017.
Mr Beattie said it has been a "huge honour" to lead the party.
"It has not been easy and at times it has been both lonely and isolating," he said.
"I am no stranger to leadership and that is how it often feels in taking a toll both physically and mentally. It also strains friendships and political relationships.
"It is now clear that some believe the momentum needed to keep the Ulster Unionist Party moving in the right direction cannot come from me.
"Irreconcilable differences between myself and party officers combined with the inability to influence and shape the party going forward means that I can no longer remain the party leader.
"Therefore, I shall stand down as the party leader and allow the party to immediately begin the process to select a new leader who may maintain the confidence of the party and continue the momentum I have started.
"It is important to acknowledge the loyalty and support from many within the party. This includes the MLA group and many other elected and non-elected members.
"I must also acknowledge that some did not agree with the direction and path I set for the party, and the vision I promoted. I hope they can see that in the long term only an inclusive Ulster Unionist Party, promoting a positive message, can secure our future.
"I hope the new leader is given the freedom to act."
Read more: Doug Beattie - a leader who struggled to lead
UUP MLA Mike Nesbitt said Mr Beattie's decision to step down came as a "shock".
Mr Nesbitt, who is also a former UUP leader, said the party faces a challenge in addressing internal matters.
Asked on RTÉ's News At One if he was sorry to see Mr Beattie go, Mr Nesbitt said: "I am because I was the man who brought Doug Beattie into the party.
"His first engagement was at a party conference when he was speaking at a fringe event.
"He was so impressive that when I got the leadership, one of my first determinations was to try and attract him in as an elected representative, and I've been very supportive of him during the years, and I'm very sorry to see him step down."
Asked if he would consider returning as leader, the Stormont Minister for Health said he had not given that possibility a "millisecond's thought".
I want to wish Doug Beattie and his family well as he announces today that he will step down as leader of the UUP.
— Michelle O'Neill (@moneillsf) August 19, 2024
We have worked constructively since he became leader of his party, including during the restoration of the executive and institutions at the beginning of this…
Northern Ireland First Minister Michelle O'Neill said she and Mr Beattie have "worked constructively since he became leader of his party, including during the restoration of the executive and institutions at the beginning of this year.
"I will continue this constructive approach with the new leader of the UUP when they take up post."
In a statement, Ms O'Neill wished Mr Beattie and his family well and said "Sinn Féin will continue to work together with all parties in the Executive and Assembly to ensure we deliver for all for our communities and our public services."
I would like to wish @BeattieDoug & his family well as he steps down as UUP leader. Doug has been a responsible and constructive leader in Northern Ireland during times of great challenge in recent years. On a personal level, I have always found him to be engaging & decent
— Simon Harris TD (@SimonHarrisTD) August 19, 2024
Taoiseach Simon Harris expressed well wishes to Mr Beattie and his family.
Mr Harris described him as "a responsible and constructive leader ... during times of great challenge in recent years.
"On a personal level, I have always found him to be engaging and decent," Mr Harris added.
Tánaiste Micheál Martin praised Mr Beattie for having emphasised "making politics work", and said that he appreciated "the constructive relationship we built".
"I know he will continue to lead by example whatever the future holds.
"A strong relationship between this government and political leaders in Northern Ireland is vital. I look forward to building the same ties with Doug's successor."
Northern Ireland Secretary Hilary Benn said Mr Beattie demonstrated "commendable leadership".
In a post on X, Mr Benn said: "I wish to thank Doug Beattie for the commendable leadership he has demonstrated over the past few years.
"It is clear that he has always sought to act in NI's best interests and I know he will continue to do so.
"I look forward to working closely with his successor."
Former tánaiste Simon Coveney - who was minister for foreign affairs during the Brexit negotiations - also paid tribute to Mr Beattie, describing him as "fair, tough when needed, reasonable and positive".
"Of course we didn't agree on everything, but Doug was someone I trusted and respected as a strong unionist leader," Mr Coveney said.
