Boris Johnson's head of policy has resigned over the PM's false claim that Sir Keir Starmer failed to prosecute serial sex offender Jimmy Savile when he was director of public prosecutions.
Mr Johnson made
the widely debunked accusation on Monday, sparking anger from MPs - including in his own party.
The PM backed down on Thursday, saying the Labour leader "had nothing to do personally with those decisions".
But Munira Mirza said he should have apologised for the misleading remarks.
In her resignation letter,
published by The Spectator, she wrote: "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."
A No 10 source confirmed Ms Mirza's resignation to the BBC.
Shortly after, No 10 sent out an email, confirming Tory MP Andrew Griffith had been named as minister for policy and head of the prime minister's policy unit in the Cabinet Office.
A Downing Street spokesman said: "We are very sorry Munira has left No 10 and are grateful for her service and contribution to government.
"[We] will work across government and with MPs as we continue to deliver and expand on our ambitious policy agenda."
Mr Johnson faced criticism on Monday where he accused Sir Keir of spending "most of his time" as director of public prosecutions (DPP) "prosecuting journalists and failing to prosecute Jimmy Savile", despite having no evidence to back up the claim.
Disc jockey and TV personality Savile was revealed to be a serial sexual abuser after his death in 2011, having attacked hundreds of women and children at various locations including hospitals, schools and the BBC.
The PM made the remark during a Commons debate on senior civil servant Sue Gray's report into Downing Street parties during lockdown, which saw him under pressure from all sides of the House over his conduct.
At Wednesday's Prime Minister's Questions, the Labour leader hit back, accusing the PM of fuelling conspiracy theories with his remarks.
But Mr Johnson replied that Sir Keir had taken "full responsibility for what had happened on his watch" as the DPP.
By Thursday, Mr Johnson appeared to have backed down, telling reporters he had not been talking about Sir Keir's "personal record" when he was DPP, adding: "I totally understand that he had nothing to do personally with those decisions.
"I was making a point about his responsibility for the organisation as a whole."
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