Sir Keir Starmer's closest parliamentary aide has resigned after she was accused of spreading false rumours about Angela Rayner's private life during a bitter briefing war over the weekend.
Carolyn Harris, who has served as Sir Keir's parliamentary private secretary since he was made leader, is accused of spreading "nasty, personal stuff" about deputy leader Ms Rayner, according to party insiders.
The departure of Ms Harris will be seen as a major blow to Sir Keir, whose authority has already been called into question over the handling of his frontbench reshuffle.
She was considered his link to backbench MPs and was among his staunchest supporters, with the two having formed a close bond since entering Parliament together in 2019.
The Labour leader's problems appeared to worsen on Tuesday night as a new poll suggested that his approval ratings had fallen significantly.
A survey from YouGov gave him a net rating of -48, with just 17 per cent of voters saying he is doing well and 65 per cent saying he is doing badly. At around the same point in Jeremy Corbyn's leadership, he had a net rating of -40 per cent.
It comes after Ms Rayner was on Sunday handed a significant promotion following a power struggle with Sir Keir, having allegedly been told she would be demoted just 24 hours earlier.
Speaking to the BBC, Ms Rayner repeatedly refused to deny claims that she had initially been sacked from her position as campaign coordinator, stating only that she was "really happy" with her new role, which includes several senior shadow cabinet portfolios.
Asked whether she agreed with Left-wing MPs who claim Sir Keir has shifted too far from Jeremy Corbyn's policies, she said: "What I heard on the doorstep is they didn't know what Keir Starmer stood for. So that's what I think our challenge is actually."
Seizing on her outmaneuvering of Sir Keir, Boris Johnson likened the Labour deputy to a "lioness", telling MPs: "The more titles he feeds her, the hungrier, I fear, she is likely to become."
It is unclear whether Ms Rayner raised complaints about Ms Harris during the talks with Sir Keir, with the deputy leader's spokesman refusing to comment.
However, multiple sources confirmed claims reported in The Times that she had separately been accused of spreading gossip about Ms Rayner to other Labour MPs. Supporters of Ms Rayner claimed the allegations were "accurate".
Concerns about Ms Harris's behaviour were then said to have been raised with Sir Keir by a senior Labour MP.
"It was nasty, personal stuff," said one shadow cabinet source. "I don't know how rogue the operation was. There is a sense that parts of the operation are making it counterproductive and hard to work cohesively [with Ms Rayner's team]."
Ms Harris was approached for comment by The Telegraph, while Sir Keir's office declined to comment.
In a statement released on Tuesday, Ms Harris said: "It has been the proudest moment of my career to co-chair the campaign that saw Keir Starmer elected as Labour leader, and to serve as his PPS [parliamentary private secretary] for the past year.
"Stepping back from this role is the right thing at this moment, coming as it does after some trying personal times and an ever-increasing workload as the deputy leader of Welsh Labour. I have enjoyed every minute, and look forward to supporting Keir the best way I can in the months ahead."