Johann Lamont took a massive sideswipe at the Westminster Labour Party as she announced she is quitting as Scottish leader.
Sources say she will formally tender her resignation to Jamie Glackin, the party’s chairman north of the border, on Saturday.
In the weeks after the independence referendum Ms Lamont said she wanted to lead Labour in to the 2016 Holyrood elections and become First Minister.
However, and despite being part of the victorious campaign for a No vote, she has decided to resign.
She is said to have become disillusioned with internal criticism of her leadership and interference from London in the running of Scottish Labour.
In a Daily Record interview, she said: “I am proud of what we have achieved over the last three years. We held Alex Salmond to account.”
But she hit out at internal Labour sniping about her leadership and criticised party bosses in London for not allowing the Scottish party more autonomy.
And she said some in Labour circles in Westminster “do not understand the politics they are facing”.
She said: “I am standing down so that the debate our country demands can take place.”
Ms Lamont believes some Labour MPs at Westminster are too concerned with their own interests.
She said: “I firmly believe that Scotland’s place is in the UK and I do not believe in powers for power’s sake.
“For example, I think power should be devolved from Holyrood to communities.
“But colleagues need to realise that the focus of Scottish politics is now Holyrood, not Westminster.”
A UK Labour party source said: “She realised it was time for a change in Scotland, she has put the party first.
“Ed (Miliband) regards her as an honourable woman who has served the party well and we hope will serve the party again in the future.”
Despite Ms Lamont playing her part in the ultimately successful Better Together movement, some MPs were critical of her performance and a perceived lack of drive to take the campaign by the scruff of the neck.
A source close to Ms Lamont told The Courier: “It’s going to kick off. She’s got a very hard message for Ed Miliband about the state of the Scottish party.
“She feels she has been inhibited in Scotland because she couldn’t do what she wanted to do in Falkirk and she couldn’t do what she wanted to do with the devolution commission.”
Her resignation will be effective immediately and her deputy, Anas Sarwar, will take over the leadership on a temporary basis until a successor is chosen.
East Refrewshire MP Jim Murphy emerged as the early favourite to take the top job and won early support to succeed, including from Dunfermline and West Fife MP Thomas Docherty.
Mr Docherty said: “I think it has to be Jim. He is the powerful candidate.”
Rumours were circling that Scottish Labour’s general secretary Ian Price had also resigned, although those were yet to be confirmed.
One source said: “He came to the decision he didn’t have the support of all senior politicians.”
Another added: “Ian had an abundance of new ideas, every one vetoed by London.”
Ms Lamont is the second Scottish party leader to resign following the referendum after First Minister Alex Salmond decided to step down from his role.
Mr Salmond will be replaced as First Minister and SNP party leader by deputy Nicola Sturgeon at their party conference in Perth next month.
Ms Sturgeon 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.
“The scale of the infighting between Scottish Labour and Labour at Westminster is exposed for all to see.
“The London-based leadership pulling the party’s strings in Scotland, and Labour campaigning side-by-side the Tories in the referendum, has proved a deeply corrosive combination which is causing Labour support to plummet in Scotland.”