MSP Neil Findlay has quit Scottish Labour’s front bench and is to stand down from the Scottish Parliament at the next election.
The Lothians MSP was the party’s Brexit spokesman and business manager, and he ran its European election campaign.
He said he had been thinking about his position for six months, and had decided to step down in March.
However, he urged the party to end its “eternal internal fighting” and “toxic culture of leaks and briefings”.
Scottish leader Richard Leonard said he was “very sorry” that Mr Findlay was stepping down and praised his “outstanding” work as an MSP.
Mr Findlay who is a close ally of Jeremy Corbyn was previously a Labour councillor in West Lothian, and has been an MSP since 2011.