James Mills, a long-serving aide to the shadow treasury team and John McDonnell, has quit his post.

Mills, who joined Jeremy Corbyn’s first leadership campaign on secondment from the Communication Workers Union, where he was working as maternity cover, before joining McDonnell’s staff in the autumn of 2015, is stepping back to move to Manchester full-time, where his wife works and his daughter is at nursery. He said his goodbyes to fellow staffers today. He is likely to write a book, though it is more likely to focus on ideas than the inner-life of the Corbyn project.

The longtime Labour staffer had already stepped back from his job as McDonnell’s chief spin doctor to spend more time with his family, working as an aide to the shadow treasury team and advising Corbyn on strategy from June of this year, but will now leave the party’s employ entirely. Andy Whitaker, who replaced him as McDonnell’s chief spinner, is likely to take on Mills’ remaining functions full time.

At the start of his time at McDonnell’s side, Mills had an easier relationship with journalists than many in the Corbyn project, having worked with several in his time as a Labour party staffer. He was not without enemies within the party, however. He was credited – or blamed – for reports of a “revenge reshuffle” of the shadow cabinet in the Christmas of 2016. (Ultimately, only Michael Dugher, the shadow culture secretary, and Pat McFadden, the shadow Europe minister, were dismissed.) At a tense meeting of shadow cabinet staffers after the reshuffle, Nick Hope, then an aide to Hilary Benn, blamed Mills for a Christmas break dominated by stories that “I was about to lose my job”.

Among journalists, some in the parliamentary press gallery blamed him for John McDonnell’s no-show at the annual press gallery dinner. But others found him a genial and helpful presence, with another member of the lobby praising him as a “straight bat” who “knew how the game worked”.

Mills was at the centre of many of the big internal battles of the Corbyn project. Along with James Meadway, McDonnell’s chief economic adviser, he was responsible for selling McDonnell’s commitment to the fiscal credibility rule – which commits Labour to balancing the books day-to-day and borrowing only for investment – and to the party’s support of the 40p tax rate. Mills is credited with coming up for the name of the rule, and his advocacy – coupled with his previous career outside the reaches of the Labour left – for the rule made him the subject of suspicion among some Corbynite officials. Recent briefings about a split in the Labour leadership, which made their way to the FT and Times, were blamed by one official in the leader’s office as the work of a “malcontent on the way out”. Friends of Mills dispute this.

But for McDonnell he will have reason to miss his most experienced aide, who helped him prepare for his first major media interviews and first devised the label that McDonnell was intending to rebrand himself as “a bank manager”. (McDonnell favoured the term “bureaucrat” for the image he wanted to create.) He was instrumental in persuading the shadow chancellor that he needed to publicly disavow his past support for the IRA, and coined the term “tea offensive” to describe McDonnell’s meetings with business leaders. It marks the departure of one of Team Corbyn's longest-serving officials, who worked near the top level of both the 2015 and 2016 Labour leadership campaigns. Given the newfound interest in all things Labour among British lobbyists, Mills may well hope that a well-paid berth in public affairs may await.