The clearout of former Vote Leave staffers in Boris Johnson’s inner circle has continued with the departure of Oliver Lewis, a key aide who was appointed to head the No 10 unit devoted to keeping the union together just over a fortnight ago. Lewis, who was head of research at the Vote Leave campaign under Dominic Cummings, acted as the de facto deputy to David Frost when he negotiated the UK’s Brexit deal. Friends said Lewis had decided to resign, as he felt his position inside Downing Street had become untenable – with one suggesting he had been accused of leaking to the press. “He was loyal to Boris for more than five years: he’s done an awful lot for the administration,” said one senior Tory. No 10 has been beset by bitter internal power struggles in recent months, most dramatically the departure in November of Cummings, Johnson’s chief aide, shortly after the resignation of another Vote Leave alumnus, the then head of communications, Lee Cain. Those departures were spurred after Cain’s promotion to chief of staff was blocked by Johnson’s fiancee, Carrie Symonds. The loss of Lewis is also likely to be seen as connected to the continued tussles for influence inside No 10. It is nonetheless a surprise, with Lewis’s position appearing cemented earlier this month when he was put in charge of the union unit, the section inside No 10 devoted to policies aimed at maintaining the union. The appointments of Allegra Stratton as Johnson’s press secretary and former civil servant Dan Rosenfield as his chief of staff were meant to mark a professionalisation of the No 10 operation, in place of Cummings’s idiosyncratic management style. But there are continued reports of tensions inside Downing Street. Lewis replaced the former Scottish Conservative MP Luke Graham shortly after Johnson made a visit to Scotland to push his pro-union message. It is unclear who will take on the critical role, with Scottish independence deemed a growing threat inside government as the Holyrood elections loom. The Scottish National party’s Westminster deputy leader, Kirsten Oswald, said: “Boris Johnson’s taxpayer-funded anti-independence campaign is completely falling apart. As support for independence grows, the Tories are losing advisers like rats on a sinking ship.” Lewis’s departure followed the unexpected promotion of Frost to a seat around the cabinet table earlier this week. Frost’s new role, in which he will also chair the partnership council overseeing the implementation of the EU trade agreement he negotiated, came after several allies of Michael Gove were given plum jobs in No 10, which reportedly irked the Tory peer. Frost had previously been earmarked by Johnson as national security adviser – an appointment openly criticised by former prime minister Theresa May because of his lack of foreign policy expertise. He was then shifted to a role as international policy adviser, focused on Brexit – but friends say he threatened to resign earlier this week, amid concerns about reporting lines in No 10. Gove’s allies have insisted he was not put out by the pugnacious Frost’s elevation, which appeared to sideline Gove from involvement in implementing Brexit, including the Northern Ireland protocol – previously a central part of his role. “David is a proven negotiatior and will sort out all outstanding issues with the protocol,” said a source close to Gove. However, Frost is regarded by his EU counterparts as a hardliner, whose elevation is unlikely to smooth fraught relations over the protocol.