Jim Murphy has announced his intention to resign as Scottish Labour leader next month - and declared war on union boss Len McCluskey.
The recently ousted former East Renfrewshire MP has made the decision after the party was almost wiped out in Scotland at the general election.
He said the party remains divided, and said he intends to use his final month as leader to prepare the ground for his successor and leave behind a stronger party rather than leaving abruptly.
Labour had been crushed between "two nationalisms" in Scotland and England, he added.
He does not intend to stand for election to the Scottish Parliament as previously planned, saying: "It's time for me to do something else."
Murphy is resigning despite winning the support of the Scottish Labour executive in today's vote of no confidence, with a tight vote in his favour of 17 to 14.
"It is clear that a small minority who didn't accept my election as leader of the Scottish Labour Party just five months ago won't accept the vote of the executive today and that will continue to divide the party," he said.
"Today I received more support in the executive vote than I did from members of the executive when I stood for election five months ago."
He has pledged to table a report of proposed reforms to the executive next month.
"When I table that report at next month's meeting of the Scottish Labour Party executive, I will also table my resignation as leader of the Scottish Labour Party.
"It will be for the party executive to decide whether it accepts the reforms proposed, but a party in such urgent need of reform blocks those changes at its peril."
He added: "The Labour Party's problem is not the link with trade unions, or even the relationship with Unite members - far from it.
"It is the destructive behaviour of one high profile trade unionist.
"One of the things about stepping down is that you can say things in public that so many people in the Labour Party only say in private.
"So whether it is in Scotland or in the contest to come in the UK, we cannot have our leaders selected or deselected by the grudges and grievances of one prominent man.
"The leader of the Scottish Labour Party doesn't serve at the grace of Len McCluskey, and the next leader of the UK Labour Party should not be picked by Len McCluskey."
Murphy added: "The siren voice from behind a big desk in Unite's headquarters in London shouldn't be allowed to instruct what the Scottish Labour Party does.
"Len McCluskey and the Unite leadership in London are the type of people who could back the wrong horse in a one horse race.
"That is just the way of the world, but it shouldn't be the way of the world in the future and that is a lesson for the entire British Labour party."
He added: "Once I stand down Kezia Dugdale, the deputy, will fulfill the role as acting leader.
"I will stand down at the next meeting of the Scottish Labour Party executive.
"But the problem that the Scottish Labour Party has had for some time is that we have had leaders who have left at a moment's notice.
"I don't question the reasons, but we know what happened. Wendy (Alexander) left abruptly, Henry (McLeish) left abruptly, Johann (Lamont) left abruptly.
"We need that period of stability over the next month so that by the time of the next Scottish Labour Party executive, I will table that report about the modernisation of the Scottish Labour Party, because to some extent the Scottish Labour Party is the least modernised part of the entire UK Labour movement and that can no longer continue."