Mayor of London Ken Livingstone has accepted the resignation of senior aide Lee Jasper.
The move follows a string of claims about Mr Jasper's alleged misuse of City Hall funds.
The Greater London Authority insisted that there was no foundation for the allegations against Mr Livingstone's adviser on race relations.
But it added that Mr Jasper had tendered his resignation after new claims.
"The mayor has accepted it," a spokesman said.
Conservative mayoral candidate Boris Johnson said the resignation of one of the Labour mayor's most senior and trusted advisers was "further proof that Ken Livingstone has had his day".
He said: "He has repeatedly said he is directly responsible for the actions of his advisers and he should take Jasper's place and answer the questions that Jasper won't.
"Otherwise Londoners can only conclude he is arrogant and out of touch. Londoners deserve to know where their hard-earned cash has gone."
Scotland Yard announced an end to its investigation into Mr Jasper's activities last month, saying there was no evidence of criminal activity.
Mr Jasper had asked Mr Livingstone to refer a string of allegations against him to police in an attempt to clear his name over alleged funding irregularities involving community projects.
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