British Prime Minister Boris Johnson was on Friday scrambling to save his premiership after four of his inner circle quit in the fallout from lockdown-breaking parties in his Downing Street office.

Long-term ally and key policy advisor Munira Mirza unexpectedly quit on Thursday, delivering a hammer blow to the embattled leader and sparking a flurry of further resignations in his top team.  

Another adviser was expected to quit on Friday, although Energy Minister Greg Hands said the exodus was a sign that Johnson was "taking charge", having promised a clear out of staff following the "partygate" revelations.

"The Prime Minister was absolutely clear on Monday that there would be changes at the top of No 10 and that is what he has delivered," he told Sky News.

But the manner of the departures, particularly that of loyalist Mirza, has increased the sense that the fate of the prime minister is no longer in his own hands.

Mirza quit after Johnson linked Labour opposition leader Keir Starmer to the failure by UK authorities to prosecute veteran TV host Jimmy Savile for child sex offences. Revelations about Savile emerged after his death in 2011.

Johnson belatedly tried to backtrack late Wednesday, after strong criticism from some of his own Conservative party MPs, sections of the media and a lawyer representing victims of Savile. 

But Mirza, an adviser to Johnson since his days as London's mayor, said that did not go far enough, according to her resignation letter reported by the Spectator magazine.

And in another sign of danger to the prime minister, his finance minister Rishi Sunak, who is tipped as a leading contender to replace Johnson, did not defend his boss's comments.

"I wouldn't have said it and I'm glad the prime minister clarified it," he told a news conference on Thursday night.

British politics and the country's media has been gripped for weeks about the "partygate" revelations, and a brewing Conservative revolt that could yet see Johnson face a vote of confidence in his leadership.

Thursday's drama prompted the right-wing Daily Mail tabloid, which is normally supportive of the government, to headline its front page "Meltdown in Downing Street", as the resignations rolled in.

Downing Street confirmed that chief of staff Dan Rosenfield was leaving, just over a year after he took on the role with a brief to professionalise Johnson's chaotic operation.

His resignation comes after a top civil servant, in a long-awaited inquiry, this week condemned "failures of leadership" over a series of parties held in violation of Covid restrictions.

Also going is Martin Reynolds -- the top civil servant in Johnson's operation -- who sent a now-notorious email in May 2020 urging Downing Street staff to "bring your own booze" to one lockdown gathering.