Alex Rowley has quit as Scottish Labour’s local government spokesman and urged Jim Murphy to stand down as party leader.
The Cowdenbeath MSP, a close ally of former Prime Minister Gordon Brown, backed Neil Findlay’s bid to run the party north of the border when the contest for the top job took place last year.
Mr Findlay resigned from the shadow cabinet at the weekend and has subsequently called for Scottish Labour to become autonomous from the UK party.
At a crunch meeting on Monday, the majority of Labour MSPs backed Mr Murphy to stay, although in his resignation letter Mr Rowley said he called that “a mistake” at the Glasgow gathering.
He wrote: “As you know, I praised the level of hard work and dedication that you brought to the campaign over the last six months and I absolutely agree that the challenges facing Labour in Scotland will not be fixed solely by a change of leadership.
“However, we have a leader in the Scottish Parliament and much of the focus of the next year will be on the Scottish Parliament and the performance of the SNP government over the last eight years in Scotland.
“I sincerely hold the view that you continuing as leader whilst not in the Scottish Parliament, and not in an elected position holding a democratic mandate, means you will become an unhelpful distraction from the real issues that Scottish Labour must focus on.
“Over the coming weeks rank and file Labour Party members must have their say on the way forward for Labour in Scotland and I want to be part of that discussion.
“It is clear from the discussion yesterday that dissent in public from the leadership view is perceived as disloyalty, but I am convinced we need a fundamental change in direction and strategy and therefore cannot sign up to your leadership as one of your shadow team.
“From an early age my memories are of my parents talking about politics and the need for working people to organise and fight for a better and fairer society and that is why I joined the Labour Party.
“It was suggested to me at the weekend that it would be disloyal to the Labour Party if I were to speak publicly on these issues. I have given that a lot of thought and consideration and I concluded that it would be disloyal and damaging to Labour were I not to speak out.
“I believe now, more than ever, that we in Scotland need a strong relevant Labour Party and we will not achieve this under your leadership therefore I have no choice but to speak out.”
It is a further blow to Mr Murphy’s position after trades unions Unite and Aslef called for him to go at the weekend and Unison prepares to consider coming out against him.
He also faces a vote of no confidence at his party’s national executive committee on Saturday after he led the party to a catastrophic defeat at Thursday’s general election, leaving Labour with just one MP in Scotland and losing his own seat.