Keir Starmer’s closest parliamentary aide, Carolyn Harris, has resigned after MPs suggested she had stoked tensions during the bitter briefing war between Starmer’s office and his deputy, Angela Rayner. Harris, who was Starmer’s parliamentary private secretary (PPS) and acts as a liaison with other MPs, resigned on Tuesday, becoming the latest casualty after a difficult reshuffle over the weekend, in which Rayner reacted angrily after Starmer attempted to move her from the position of party chair. The Guardian understands a formal complaint was lodged by a veteran MP after they became concerned about Harris’s conduct and comments she made about the private lives of shadow cabinet ministers that are said to be baseless. The complaint was made shortly before last week’s elections but concern from MPs escalated following the row with Rayner. “People have been saying to Keir for some time, this is not the PPS you should be having,” one senior MP said. In a statement released by Starmer’s office, Harris, who is MP for Swansea East and also Welsh Labour’s deputy leader, 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 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 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.” Harris and Starmer’s office did not comment on suggestions she had been sacked for her conduct over the reshuffle. Harris has been a vocal champion of Starmer in MPs’ WhatsApp groups, particularly as relations soured with some of his key backers over the decision to vote in favour of Boris Johnson’s Brexit deal. Rayner’s spokesperson refused to comment on the matter and said she was focused on working constructively with Starmer’s office on Labour’s response to the Queen’s speech. MPs said they believed Harris was not acting at the behest of Starmer. “This is the opposite way to how he operates – which has clearly been naive – but she was freelancing,” said one shadow minister. “Whoever you put in charge, they have to be mindful that MPs are not your employees,” they added. “You cannot fire people like they are paralegals. You are not the boss. We have our own mandates and the best power is persuasion, not lording over people.” One frontbencher said Harris’s relations with the parliamentary Labour party (PLP) had been deteriorating for some time, leading Starmer’s office to moot the possibility of appointing a second PPS alongside her. However, Harris is said to have been unwilling to share the role. MPs speculated on Tuesday that Starmer could appoint a new PPS from the left of the party. One said: “That would be the best solution – especially as there are a lot of promising new women of colour who are ambitious. Keir badly needs to diversify his team. “But given their record so far, they have zero political acumen to make an appointment like that, so they probably won’t.” Appointing an MP of colour to the key role would also help address some anxieties from the BAME Labour group of MPs, who have been riled by the dismissal of Valerie Vaz as shadow leader of the House of Commons. Her replacement, Thangam Debbonaire, is also a woman of colour, but she has been replaced in her former role as shadow housing secretary by Lucy Powell. Some MPs speculated Harris’s dismissal could be the beginning of a wider shake-up of Starmer’s team, after widespread disgruntlement about the result of the Hartlepool byelection, linked to the decisions made by his most senior aide, Jenny Chapman, the former MP for Darlington. The Labour leader hired Deborah Mattinson, the director of Britain Thinks and Gordon Brown’s former pollster, as his director of strategy last week, a move welcomed by many in the PLP. “Ultimately they need someone else to sharpen up the communications, someone who can restore his authority,” one MP said. “He looks so inept. He needs an Alastair Campbell or a Seumas Milne type figure.” Rayner told ITV news on Tuesday that her row with Starmer had not been about personal ambition, but said she had radical ideas to reshape the party’s message. Rayner said she had not engaged in briefing wars herself but did not deny she had delivered strong words in discussions with Starmer. “Everyone knows I’m quite a trade union negotiator, and a robust talker, and Keir is very frank with me and I’ve really enjoyed the fact we’ve had that constructive open relationship and I’m pleased that will continue,” she said. “Reshuffles are always a difficult process for any political party, I see it as my talents and everyone acknowledges my talents are taking it to the Tories and being me … there’s been briefings going on, I’ve not done any of that, but what I can say is I do have insecurities about ‘am I good enough?’ – I think it does come back to my working-class background.” Rayner said her current ambition was to be deputy prime minister – though it is a role Starmer would need to create if he entered No 10.