Johann Lamont will stand down as leader of the Scottish Labour Party.
The news comes just weeks after Ms Lamont had said she wanted to lead the party in to the 2016 Holyrood elections and become first minister.
Despite being part of the successful No campaign, Ms Lamont is said to be stepping down amid criticism of her leadership.
Earlier this week, two former Labour first ministers, Lord McConnell and Henry McLeish, suggested the party was in need of an overhaul.
Ms Lamont, who became party leader in 2011, is said to be disillusioned with the interference of the UK Labour Party in the party’s affairs.
A Scottish Labour source said it was a shame but not surprising that Ms Lamont had decided to stand down because she had been repeatedly undermined by senior figures within the movement.
They said the Glasgow Pollock MSP was “fed up” because she wasn’t getting support from people in key positions.
The source said Ms Lamont had been under a lot of pressure in the weeks since the referendum about the direction of travel for the party and did not want to resign. But certain people were determined to force her out.
MP Anas Sarwar, the deputy leader of the Scottish Labour Party, will take over the post on a temporary basis until a successor is chosen.
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.”
Ms Lamont has represented the Glasgow Pollok MSP constituency since 1999.
She 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.