John McDonnell has accused the group of moderate Scottish Labour MSPs who have urged their leader to resign of trying to "climb a few inches further up the greasy political pole" while people are dying.
In an outspoken assault on the campaign to oust Richard Leonard, Mr McDonnell, the former Shadow Chancellor, said Scots were "suffering, dying and losing their livelihoods" during the pandemic.
But he said that was "immaterial" to the "plotters" trying to force Mr Leonard out and claimed the Scottish people would not forgive them for "putting their own jobs and careers ahead of the jobs" of workers threatened by Covid-19.
In a direct message to the Labour politicians who have urged Mr Leonard to step down, amid dismal poll ratings, Mr McDonnell said: "My advice is get real. Get a sense of proportion."
He added: "Now is the time to put personal political career ambitions to one side and work together as a team to support Richard in making sure that the Covid pandemic is tackled effectively in Scotland and people's income and jobs are protected."
His incendiary intervention came ahead of Tuesday's weekly meeting of the Scottish Labour group at Holyrood, the first since four MSPs broke ranks last week to urge Mr Leonard to quit.
Moderate MSPs refused to respond to Mr McDonnell's comments ahead of the meeting, which is expected to be highly fractious.
The Left-winger caused fury in the group last year when he promised a future Corbyn Government would not block a second independence referendum. Labour lost six of its seven MPs in December's snap election.
However, a Scottish Labour source said: "Nobody did more damage to Scottish Labour last year than John McDonnell and the only thing members want to hear from him is an unreserved apology.
“There is only one person putting their personal ambitions ahead of the party and that’s Richard Leonard. He should do the honourable thing and resign before he inflicts even more damage than McDonnell did.”
The party has suffered appalling electoral results under Mr Leonard and insiders have warned that "without some kind of revival of Scottish Labour, the Union is in peril."
A weak Labour is expected to help the SNP win more Left-wing votes next May, boosting Nicola Sturgeon's hopes of a Holyrood majority and a second independence referendum.
The First Minister last week unveiled plans for a new Independence Referendum Bill, complete with timetable, to be published ready for the next parliament.
Former Shadow Chancellor @johnmcdonnellMP is introduced, thanking @LabourCfS for organising.
Praises @LabourRichard, says he's been at the heart of much of policy making for over a decade and warns that people "seem to forget" the commitment he has shown to progressive causes.
— Niall Christie (@NiallChristie1) September 7, 2020
MSPs James Kelly, Jenny Marra, Daniel Johnson and Mark Griffin urged Mr Leonard to resign last week, warning that he is leading the party to an electoral catastrophe.
Four Scottish Labour grandees and Rachel Reeves, Shadow Chancellor of the Duchy of Lancaster, also urged him to consider his position but he refused to quit.
But, speaking ahead of an online Campaign For Socialism rally last night, Mr McDonnell attacked those "spending their time on internal political manoeuvres" during the pandemic.
He said: "People are suffering, dying and losing their livelihoods but for these plotters that's all immaterial and what counts is their ability to climb a few inches further up the greasy political pole.
"The Scottish people will not forgive these plotters for putting their own jobs and careers ahead of the jobs and livelihoods of workers under threat from the recession."