Keir Starmer has faced down objections from Labour leftwingers to secure a change in the way members of the party’s ruling national executive committee are elected. At an explosive meeting on Tuesday, Starmer was also confronted directly about a BBC Breakfast interview in which he described Black Lives Matter as a “moment”, and dismissed calls to “defund the police” as “nonsense”, the Guardian understands. The national executive committee (NEC) agreed by 19 votes to 12 to introduce a single transferable vote (STV) system for its CLP (constituency Labour parties) section, which represents grassroots members. The change is a relatively modest one, but it underlined the speed at which Starmer has seized control of the levers of Labour party machinery. It took Corbyn many months to secure a stable majority on the NEC – and more than a year before his pick for general secretary, Jennie Formby, was in post. Councillor Tom Miller, co-chair of the Open Labour campaign, which had been calling for the change, said, “this is a long overdue reform, that brings NEC elections in line with other major Labour party elections such as leadership contests. This will ensure members’ votes count.” In another sign of the transformation wrought in the party under Starmer’s leadership, it emerged that Seumas Milne, Corbyn’s most senior adviser and an executive director of the party, is leaving his Labour post. Friends believe he is planning to write a book. Asked about Milne’s departure, a Labour spokesman said, “we don’t comment on staffing matters”. Starmer and his allies believe the change to a proportional voting system, which had the support of the Electoral Reform Society, will dilute the power of “slates” of rivals from either wing of the party. But it was fiercely resisted by leftwingers on the NEC, amid simmering tensions in the wake of Starmer’s sacking of Rebecca Long-Bailey last week, over claims of antisemitism. One NEC member, Huda Elmi, from campaign group Momentum, said the NEC had “decided to wage war on member democracy”, calling the change a “stitch-up”. A group of NEC members, including Unite’s assistant general secretary Howard Beckett, are threatening to launch a legal challenge against the decision, which they say constitutes a “rule-change”, and should be put to a full Labour conference for approval. With this year’s gathering cancelled, that could potentially delay any change for more than 12 months. NEC members were also told by the party’s new general secretary, David Evans, that Labour’s membership is 580,000 – as high as the level reported during Corbyn’s leadership. During the meeting, which lasted more than six hours, Starmer and his deputy, Angela Rayner, were repeatedly challenged by left-wing NEC members over the switch to STV and other fraught issues such as his comments on Black Lives Matter. Starmer’s allies have insisted he was using the word “moment” in the sense of “epoch”, to emphasise the significance of BLM – but some party activists and staff complained that the remarks appeared to suggest it was a passing phase. Challenged about what he meant at the NEC meeting, the Labour leader spoke for four to five minutes, but declined to engage in an argument, NEC sources said. One person present described his response as “sucking the air out of the room”. Separately, 13 NEC members wrote to Starmer and Rayner in advance of the meeting, claiming that the party’s press office had dismissed concerns about potentially discriminatory behaviour by Labour staff revealed in a leaked report. The 860-page internal report, which emerged in April, described what it called an “abundant evidence of a hyper-factional atmosphere prevailing in party HQ in this period, which appears to have affected the expeditious and resolute handling of disciplinary complaints”. It included hundreds of private WhatsApp exchanges between named members of Labour staff, discussing Jeremy Corbyn and his close aides in insulting terms. The report’s contents, and the leak itself, are now being investigated by an independent review, chaired by Martin Forde QC. In a the strongly worded letter, seen by the Guardian, the 13 NEC members demanded an explanation of comments released to the website Open Democracy by the party’s press office. The comments appeared to dismiss as “po-faced” concerns about insulting WhatsApp messages revealed in the report, stressing they were private communications. In their letter to Starmer, the NEC members said: “The comment from the Labour party defends racist, sexist and abusive language in the WhatsApp groups, and denies that these messages between then senior party staff display a discriminatory culture within the Labour party.” Signatories included Howard Beckett, Mick Whelan, the secretary general of Aslef, and the NEC’s chair, Andi Fox. A Labour party spokesperson insisted the comments actually formed part of a statement from Labour’s lawyers, responding to claims made by Open Democracy about a specific member of staff, which should not have been sent via the press office. “For the avoidance of doubt, we are clear that the comments that have been quoted do not in any way represent the party’s position in relation to the contents of the leaked report overall and do not prejudge the outcome of those investigations,” the spokesperson said.