Andrew RT Davies announced his intention to stand down as leader of the Conservatives in the Senedd after narrowly surviving a vote of no confidence.
Mr Davies won the vote among the Tory group, 9-7, but offered his resignation after accepting his position as leader had become untenable.
In a letter to Bernard Gentry, chairman of the Welsh Conservatives, he wrote: "Last week, a group of Senedd members approached me, threatening to resign their positions in the shadow cabinet if I did not agree to step down as leader."
"I therefore requested a motion of confidence in my leadership to be held at a meeting this morning. This vote has now taken place."
"It was clear from the result that a substantial minority of the group do not support our approach, despite it being the only viable strategy available. While I would have been honoured to continue as leader, my position is consequently untenable."
'Direct contrast'
Mr Davies hit out at members of his own party for publicly contradicting and undermining the party's position.
He said: "I would like to take this opportunity to offer my heartfelt thanks to those Senedd members who did support me and all our hardworking staff.
"Similarly, I would like to thank our dedicated party members and grassroots supporters, who are the lifeblood of our party."
"Without their commitment to paying their membership fees, knocking doors, delivering leaflets and raising funds, we would be unable to further the conservative cause."
"They do so not out of financial motivation or in the interests of personal gain but because they believe passionately in conservative values and in the UK."
"This stands in direct contrast to some Conservative Senedd members."
The new Tory leader will have less than 18 months at the helm before the Senedd election in May 2026, which will see the Welsh Parliament expand from 60 to 96 members.
'Privilege'
Mr Davies, who has led the Tories in two stints covering most of the past 13 years, will officially stand down once his successor is elected.
He wrote: "Moving forward, it is clear the Welsh Conservative Party must decide exactly what it stands for. This is a decision in which everyone must be given their say."
"It is therefore imperative that a contest is held to elect my successor, with party members given the opportunity to choose."
"There must be a healthy debate in which the candidates, their ideas and qualities are placed under robust scrutiny. To confirm, I will not be standing in this contest."
"Once again, I would like to take the opportunity to thank the Welsh public and party members for giving me the opportunity to serve. Public service is a privilege and I will remain forever grateful for being given the chance to play my part."
Reacting to his resignation, Plaid Cymru leader Rhun ap Iorwerth said: "The legacy of the Tories in Wales is one of chaos and cuts - a legacy that was roundly rejected at the ballot box this year."
"They offer no solutions for the challenges facing our communities and nor do they have a serious plan for government."
"Plaid Cymru stands ready to offer Wales a fresh start. While the Tories fight amongst themselves and Labour continues to let down our communities, Plaid Cymru is united and focused on delivering our vision to rebuild our economy, fix the NHS, demand fair treatment from UK Government and show real ambition for the future of our nation."