Scottish Labour leader Johann Lamont has said that her resignation as leader comes after senior members of the party “questioned” her role in a new phase for the organisation.
Ms Lamont said she was stepping down so that “real discussion” could take place about the future of Scottish Labour.
She has accused some in the UK party of treating Scotland as a “branch office” and called for greater autonomy north of the border.
Speculation has now begun over who will replace her, with f ormer prime minister Gordon Brown MP, Jim Murphy MP and interim leader Anas Sarwar MP all linked to the leadership role.
In her resignation letter to Jamie Glackin, chair of the Scottish Labour Party, Ms Lamont said: “It has been an immense privilege to lead our party during this momentous time in Scottish politics.
“You will know the significant victory we campaigned for in the referendum has opened a new chapter in the debate about the future of the Scottish Labour Party.
“Some, including senior members of the party, have questioned my place in this new phase. In order that we can have the real discussion about how we take Scottish Labour forward, I believe it would be best if I took myself out of the equation and stepped down as leader.
“Anas Sarwar will take over as leader in the interim while the party elects my successor. He has been a fantastic deputy to me and I trust he is more than capable of leading the party through this transition.
“Despite the serious challenges we face, I strongly believe that the Labour Party is not only our best chance of achieving a better, fairer Scotland, it’s our only chance.”
Labour former first minister Lord McConnell said UK party leader Ed Miliband had questions to answer, while his predecessor Henry McLeish said the party faced a problem of ”historic, epic proportions”.
First minister-in-waiting Nicola Sturgeon said the resignation revealed Labour to be in “complete meltdown” in Scotland.
Michael Connarty, Labour MP for Linlithgow and East Falkirk, has backed Mr Brown for the job as ”a towering figure” who was ”speaking the language of the people of Scotland”.
He told BBC Radio Scotland’s Good Morning Scotland programme: ”We should be talking about Gordon and Gordon alone. I’ll be seeking him out and so will other people.”
Mr Murphy had previously ruled himself out of mounting a challenge to Ms Lamont and urged the party to unite around her last weekend.
Mr Sarwar praised the former leader’s “selflessness” in standing down, adding: ”The Scottish Labour Party will now consider and set out in due course the process for electing a new leader as we continue to work to return a Labour government at the general election in 2015 and in the Scottish Parliament elections in 2016.”
Glasgow MSP Ms Lamont had pledged to lead Labour into the 2016 Holyrood elections despite questions over her political future in the wake of last month’s Scottish independence referendum, in which large numbers of Labour supporters voted to leave the UK.
Announcing her resignation, Ms Lamont told the Daily Record: ”Just as the SNP must embrace that devolution is the settled will of the Scottish people, the Labour Party must recognise that the Scottish party has to be autonomous and not just a branch office of a party based in London.
”Scotland has chosen to remain in partnership with our neighbours in the UK. But Scotland is distinct and colleagues must recognise that.
”There is a danger of Scottish politics being between two sets of dinosaurs … the Nationalists who can’t accept they were rejected by the people, and some colleagues at Westminster who think nothing has changed.”
She attacked those who had attempted to undermine her position as she sought to reform the party in Scotland.
Moves to replace Scottish Labour general secretary Ian Price without her consultation appear to have been the final straw.
Mr Miliband praised her contribution to the referendum campaign.
He said: ”She campaigned the length and breadth of Scotland making the case for social justice within the United Kingdom. She has led the Scottish Labour Party with determination. I know she will continue to serve her constituents.”
Ms Sturgeon highlighted polling analysis putting Labour at 26% for next year’s UK general election, compared with SNP support of 43%.
She said: ”Johann Lamont carries my personal best wishes, including in continuing to represent the people of Glasgow Pollok, but there is no question that her shock resignation reveals Labour to be in complete meltdown in Scotland.
”Labour were already a party in crisis, and Johann Lamont’s resignation – caused by infighting and deep division – has plunged them to a new low.”
First Minister Alex Salmond said: ”It was always very clear that Johann Lamont was never able to be meaningfully in charge of Labour in Scotland, and that is laid bare in dramatic fashion in her resignation comments.”
Mr Brown issued a short statement that read: “I am sorry to hear that Johann has resigned.
“She brought determination, compassion and a down-to-earth approach to the leadership and deserves great credit for taking on the challenge after 2011.
“I wish her well in the future.”
Pat Rafferty, Scottish secretary of the Unite union, said: “Johann Lamont’s departure must now spark the desperately needed process of renewal of Scottish Labour’s relationship with the people of this nation.
“Unite urges the party to take full and honest consideration of the recognisable concerns raised by Ms Lamont.
“Labour in Scotland must understand that there has been a seismic change in the political landscape in our country. People want a party that grasps this and has the vision to work with them to build the fairer nation they desire.”
Unite has 165,000 members in Scotland.