Mr Johnson’s attempt to backtrack was not enough for his close aide Munira Mirza, who quit No 10 over his refusal to apologise for the “scurrilous” attack on Sir Keir.

Chancellor Rishi Sunak, viewed as a potential successor to Mr Johnson, said he would not have made the comment.

What are the claims and where did they come from?

The claims initially surfaced online on Facebook and Twitter in 2020, following Sir Keir’s election as Labour leader.

Sir Keir was Director of Public Prosecutions and head of the CPS from 2008 until 2013 during the time in which the CPS failed to prosecute Jimmy Savile, who was revealed to be one of the UK’s most prolific sex offenders.

During this time, four allegations of sexual assault by Savile in the 1970s were made to Surrey and Sussex Police in 2007.

Savile was interviewed under caution in 2009, and the police consulted the CPS for guidance about the allegations.

Was Keir Starmer responsible for the CPS decisions?

Despite being the head of the CPS, Sir Keir was not the lawyer responsible for the cases.

The reviewing lawyer for those cases advised that no prosecution could be brought on the grounds that none of the complainants were “prepared to support any police action”.

In 2012, Sir Keir appointed Alison Levitt QC, his Principal Legal Advisor, to examine the decisions.

Her report, published in 2013, found that Savile might have been prosecuted if police and prosecutors had taken a “taken a different approach”.

She said that the allegations were “both serious and credible; the prosecutor should have recognised this and sought to ‘build’ a prosecution”.

She said that the police treated the victims’ accounts of abuse with “a degree of caution which was neither justified nor required”.

Despite these failures, Ms Levitt said that she had seen nothing to suggest the decisions not to prosecute Savile were “consciously influenced by any improper motive on the part of either police or prosecutors”.

The 129-page report makes no reference to Sir Keir’s involvement in any of those decisions.

Sir Keir criticised the CPS’s handling of the cases.

He said: “I would like to take the opportunity to apologise for the shortcomings in the part played by the CPS in these cases. If this report and my apology are to serve their full purpose, then this must be seen as a watershed moment.”