A director of the Jeremy Corbyn-backing pressure group Momentum has taken charge of Labour's disputes panel in a move which critics said showed the party was being controlled by a "leftist clique".
A source confirmed that Christine Shawcroft had beaten Ann Black in a vote of members of Labour's national executive committee (NEC) a day after a slate of three left-leaning candidates, including Momentum founder Jon Lansman, were elected to the ruling body.
Her election was seen as further evidence of the Labour leader tightening his grip on the party's internal structures.
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".
Richard Angell, director of the Blairite pressure group Progress, said Ms Shawcroft had previously been suspended from Labour for supporting former Tower Hamlets mayor Lutfur Rahman, who was barred from running for office for five years after being found guilty of corrupt and illegal practices.
"The irony of someone who was kicked out of Labour because she supported Lutfur Rahman, the disgraced former mayor of Tower Hamlets, becoming the chair of the disputes committee shows the contempt the Momentum leadership have for due process and party democracy," Mr Angell said.
The disputes panel looks into allegations of abuse and sexual harassment, as well as other issues, and operates in a quasi-judicial fashion.
As chair, Ms Shawcroft will also become one of the nine officers of the NEC.