The head of Labour's disciplinary panel has been ousted and replaced with a director of the pro-Jeremy Corbyn campaign group Momentum.
Today's vote to remove Ann Black comes a day after three left-wing activists won new seats on the party's ruling National Executive Committee (NEC) by a landslide.
A source confirmed reports that Ms Black was replaced as chair of the party's Disputes Panel with Christine Shawcroft by 22 votes to 15 this afternoon.
Ms Shawcroft, a director of Momentum Campaign (Services) Ltd, last year described Mr Corbyn's Labour opponents as "hard-right" and deputy Labour leader Tom Watson as "rather right wing".
She has been on the NEC for more than a decade but was suspended from the party in 2015 for publicly supporting disgraced Tower Hamlets mayor Lutfur Rahman.
Although the panel - a sub-group of the NEC - is designed to be independent, it has a crucial role analysing complaints against members accused of sexism, racism or anti-semitic abuse.
Cases the panel has presided over include those of Simon Danczuk, who was banned from standing as Labour's candidate in 2017 over lewd texts he sent to a 17-year-old girl.
Young Labour NEC rep Jasmin Beckett said she was "disgusted" by the removal of Ms Black and added: "The NEC Disputes panel is a vital body... not a factional playground."
Richard Angell, director of Blairite pressure group Progress, accused Momentum of "contempt for due process and party democracy".
Blackadder star and Labour member Tony Robinson accused supporters of Mr Corbyn of "undermining" Ms Black.
Before the result was announced, he tweeted: "Ann Black's brilliant- a radical voice for ordinary members on the NEC with an awesome work rate. The idea of the Corbynite left trying to undermine her is outrageous. If true it would confirm what I've always feared- our party completely taken over by and for a leftist clique".
But Ms Shawcroft told the Mirror: "I really don't know what all the fuss is about".
Ms Shawcroft denied the move was a "left-wing takeover", saying she had been on the NEC for 19 years and the Disputes Panel was "just a minor subcommittee".
She added: "The centre left grassroots alliance, which supports the nine [local party] NEC members, represents the mainstream of the party."
She defended her comments about Tom Watson, saying: "I would say Tom is right of centre, I don't mean it as an insult.
"I would imagine he would agree that that is his position."
And she stuck by her defence of Lutfur Rahman, saying his case - in which he was found guilty of corrupt and illegal practices by an electoral court in 2015 - was "a terrible miscarriage of justice".
A source said Jeremy Corbyn did not attend today's meeting where the vote took place or take part in the vote for a new chair.
The Labour leader's office declined to comment.
It comes amid a new row between self-described 'moderates' and left-wingers who have clinched power on Labour's ruling body.
A landslide election result yesterday tipped the National Executive Committee (NEC) more or less in favour of Jeremy Corbyn and his left-slate allies.
This could make it easier to force MPs to face mandatory reselection.
Prominent Jeremy Corbyn backer Paul Mason declared: "Just like when we go to the coffee shop and get to choose between latte, cappuccino and tea, I want once in every five years to have it hanging over every lawmaker that they could be deselected if they don't do their job properly."
But anonymous MPs warned the Times and Daily Mail they would quit the party's grouping in the Commons and sit as independents if they are deselected by their local members.
Ms Shawcroft warned last year there was a "democratic deficit" in Labour and the “structures of the party don’t reflect the support for Jeremy Corbyn that has been shown by the mass of the membership".
Momentum members must all now join the Labour party after the group, which began as a campiagn for Jeremy Corbyn's leadership, was initially dismissed as a "party within a party".