A long-standing aide of UK Prime Minister Boris Johnson has resigned, citing his 'baseless' attack on opposition leader Sir Keir Starmer over his record as director of public prosecutions (DPP). 

Munira Mirza has worked with Johnson for 14 years, during his time as Mayor of London and later Prime Minister. Until Thursday she was Head of Policy at No.10 Downing St. 

But she said his comments in the House of Commons accusing Starmer of failing to prosecute Jimmy Savile, a TV personality who was found after his death to have raped and sexually abused children for decades were 'inappropriate.'

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In a letter published via political magazine The Spectator Mirza said "there was no fair or reasonable basis for that assertion."

While it is true Starmer was DPP at the time allegations around Savile were beginning to circulate, Starmer was not responsible for the decision not to prosecute. 

"This was not normal… politics, it was an inappropriate and partisan reference to a horrific case of child sex abuse," she added. 

On Thursday morning Johnson made a statement qualifying his position, but as Mirza wrote: "despite my urging, you did not apologize for the misleading impression you gave." 

The letter and the accusation are not an isolated incident, coming as the internal party arguments about whether or not Johnson should continue to lead the country following the "partygate" scandal increasingly place pressure on the PM. 

Three lawmakers from his own party made public calls for him to resign on Wednesday, following similar demands from other MPs.