Boris Johnson was braced for further departures of senior aides from his Downing Street team on Friday as some government ministers were said to be considering their position.

Four of the UK’s prime minister most prominent aides left Number 10 on Thursday, including Munira Mirza, Johnson’s policy unit chief and a close ally for almost 15 years, who denounced his claim that Labour leader Sir Keir Starmer had failed to prosecute the late sex offender Jimmy Savile.

Allies of Johnson said he seized upon Mirza’s departure to begin “a much needed clear out” of Downing Street that was promised to Conservative MPs following the release of a critical report into a series of parties that broke lockdown rules.

Elena Narozanski, a member of the Number 10 policy unit, resigned on Friday morning. No reason was given for her departure.

Those close to the prime minister, however, are concerned that some ministers may resign, moves that would further destabilise Johnson’s position after chancellor Rishi Sunak publicly criticised his conduct.

“More people will leave Number 10, it’s not over,” said one ally.

Rather than defending Johnson, Sunak used a Downing Street press conference to describe Mirza as a “valued colleague” while undermining the prime minister by saying: “Being honest, I would not have said it.”

Lord Gavin Barwell, who served as Theresa May’s Downing Street chief of staff, said on Friday that some ministers were considering their positions following Mirza’s departure.

“Large numbers of government ministers are deeply, deeply uncomfortable with the way the prime minister is behaving. Lots of people are considering the dilemma Munira faced,” he told the BBC.

Dan Rosenfield, Johnson’s chief of staff, resigned after failing to get a grip on a chaotic Number 10 operation, which is riven with factions. Jack Doyle, director of communications, has also quit, along with Martin Reynolds, Johnson’s principal private secretary.