Mayor of London Sadiq Khan has accused Boris Johnson of launching the Jimmy Savile slur on Sir Keir Starmer so "people start believing it".
Speaking on LBC, Mr Khan said the Prime Minister was "amplifying right wing conspiracy theories" to try and damage the Labour leader as part of a "dead cat" strategy to distract the public from Partygate.
Sir Keir has been the subject of death threats after the PM made false claims in parliament about him being personally responsible for the Crown Prosecution Service's failure to prosecute the paedophile.
He was also mobbed by protestors repeating the slur and had to be escorted to safety by police officers.
The Mayor said the PM had a "clear tactic" of deliberately misleading the public at a time when Downing Street was under pressure over rule-breaking parties.
He said: "And I make this point in relation to the Prime Minister's conduct: amplifying right wing conspiracy theories not only mainstreams it, not only normalises it, it gives succour to some of these people on the fringes to become bolder.
"And also what it does is it deliberately muddies the water. It's a dead cat that serves two purposes.
"One is the more we repeat these slurs and these lies, the more becomes normalised and people start believing it but secondly distracts from the really serious issue about the Prime Minister who makes the rules, breaking the rules. That was the clear tactic from Boris Johnson."
The Mayor said cops were reviewing Sir Keir's security in the wake of the row, adding: I've not spoken to [Keir's] wife, or his family or his friends, but just think about the distress it's causing them."
He added: "He's at risk and there's a link I'm afraid between the death threats he's received and the language used by the Prime Minister. We can't escape that there's a cause and effect."
The Prime Minister also faced a backlash from Tory backbenchers over the smear, with former Chief Whip Julian Smith among those urging him to withdraw the comments.
The claim was made during a testy Prime Minister's Questions in the Commons as, when pressed over the Met's Partygate inquiry, Mr Johnson said as director of public prosecutions from 2008 2013, Sir Keir had “spent most of his time prosecuting journalists and failing to prosecute Jimmy Savile”.
He later rowed back from the remarks, however, telling broadcasters: “I want to be very clear about this because a lot of people have got very hot under the collar, and I understand why.
“Let’s be absolutely clear, I’m talking not about the leader of the opposition’s personal record when he was director of public prosecutions and 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. I really do want to clarify that because it is important.”
