Downing Street parties: Boris Johnson in crisis as two aides resign as Rishi Sunak criticises PM's Jimmy Savile smear of Keir Starmer

Boris Johnson's leadership was further thrown into crisis by the resignation of two key aides as Chancellor Rishi Sunak criticised the Prime Minister during a broadcast from No 10.

Munira Mirza, who had been one of the PM's most loyal and longstanding advisers, quit over his use of a "scurrilous" Jimmy Savile smear.

She told the Prime Minister she was departing as No 10's head of policy because he refused to apologise for the widely criticised attempt to tarnish Sir Keir Starmer.

Then came the resignation of Jack Doyle, No 10's director of commutations who was embroiled in allegations of lockdown-breaching parties in Downing Street.

Downing Street head of policy Munira Mirza. Picture: Mary Turner/Getty Images

In a live broadcast on the cost-of-living crisis, Mr Sunak praised Ms Mirza as a "valued colleague" and criticised the Prime Minister's Savile remarks, saying: "I wouldn't have said it."

Earlier in the day, the Chancellor did not rule out a leadership bid if Tory MPs force out Mr Johnson over allegations of lockdown-breaching parties in Downing Street.

Mr Johnson backtracked on the debunked claim that the Labour leader failed to prosecute Savile while director of public prosecutions (DPP).

But Ms Mirza, who first advised him as London mayor more than a decade ago, said she was quitting after the Prime Minister stopped short of giving the apology she demanded.

"I believe it was wrong for you to imply this week that Keir Starmer was personally responsible for allowing Jimmy Savile to escape justice," a letter seen by The Spectator magazine read.

"There was no fair or reasonable basis for that assertion. This was not the usual cut and thrust of politics; it was an inappropriate and partisan reference to a horrendous case of child sex abuse. You tried to clarify your position today but, despite my urging, you did not apologise for the misleading impression you gave.

"You are a better man than many of your detractors will ever understand which is why it is so desperately sad that you let yourself down by making a scurrilous accusation against the Leader of the Opposition."

Mr Johnson, who once praised Ms Mirza as a "brilliant thinker" and listed her as one of the five women who had influenced and inspired him the most, said he was "sorry" to lose her.

But, in an interview with Channel 5 News, Mr Johnson disagreed that his Savile smear was inappropriate.

Tory MP Andrew Griffith was swiftly appointed to fill her role.

Mr Doyle gave a resignation speech to staff in No 10 saying "recent weeks have taken a terrible toll on my family life" but he always intended to resign after two years in the role, according to the Daily Mail, who he used to work for.

The former journalist reportedly attended at least two of the 12 events in Downing Street and wider Government that are under investigation by police.

A No 10 spokesman said: "Jack Doyle has left government. He has made a huge contribution and the Prime Minister is immensely grateful for the work he has done."