Scottish Labour's Euro election campaigns chief Neil Findlay has announced he is to quit the party's front bench and will stand down as an MSP at the next Holyrood vote.
Findlay hit out at "internal battles" within Scottish Labour as he unveiled his shock departure.
The Lothians MSP, who was elected in 2011, is an influential figure in the party and has been a leading ally of Jeremy Corbyn at Holyrood having headed up both his leadership campaigns north of the border.
He was campaign manager for the party's European election push north of the border and criticised by some in the party for the poor performance which saw Labour wiped out in Scotland. But he said today that his decision to stand down was made in March and had been under consideration for the past six months.
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He announced today that he has resigned as Scottish Labour’s Business Manager, Party Liaison and spokesman on Constitutional Relations. He has also confirmed that he will leave the Scottish Parliament at the 2021 Holyrood elections.
Speaking on Twitter he said: "Life is too short to be involved in endless internal battles with people who are supposed to be on the same side."
In a scathing resignation letter to party letter Richard Leonard he warns this could undermine the rebuilding of the party in Scotland.
The Lothians MSP calls for an "end to the eternal, internal fighting within our party and the toxic culture of leaks and briefings that come from some within the Scottish and UK Parliamentary groups.
"This self-inflicted harm has to stop - our party members and our leadership teams deserve so much better.
"Scottish Labour will win again when the focus is on the public and not the internal politics of the parliamentary group."
MPs Ian Murray and Martin Whitfield launched a coruscating attack on the party's EU election campaign following the result which saw Labour lose the two Scottish seats it took in 2014.
During his time in Parliament, Findlay led campaigns to retain the Children’s ward at St John’s hospital, supporting the blacklisted construction workers, the mesh campaign, miners’ justice and support for the victims of undercover policing amongst many others.
“I will continue to work hard and diligently on these and other issues right up until I leave parliament,” he added.
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Scottish Labour Leader, Richard Leonard said said today he was sorry to lose Mr Findlay from the party's front bench.
But he said: "I also know from our conversations that this is the right decision for you personally and in that you have my full support.
“Your service as an MSP has been outstanding. You have served the communities you represent with dedication and on the basis of your strongly-held principles.
"Your campaigns, such as the mesh campaign, the pressure for an inquiry into the policing of the miners’ strike, and blacklisted construction workers are a model of what being a member of this Parliament should be about. Through this work you have demonstrated the important lesson that for socialists the purpose of the Parliament is not to be found in the daily grind of procedural arguments, but instead in how the Parliament relates to the lives of working class people in this country."