An adviser to Boris Johnson quit the British Government a few days into the job after amid controversy over his claims that white Americans are more intelligent than their black compatriots.
Andrew Sabisky, who was hired to work in Johnson's Downing Street office last week, said he had joined the government to do "real work" but found himself "in the middle of a giant character assassination."
"The media hysteria about my old stuff online is mad but I wanted to help HMG (Her Majesty's Government) not be a distraction," Sabisky wrote on Twitter. "Accordingly I've decided to resign as a contractor."
A spokeswoman for the prime minister's office confirmed that Sabisky had resigned but declined to comment further.
Sabisky became the focus of intense criticism today, with calls for Johnson to fire him. The row came after a posting on an academic blog in 2014 came to light, in which he said: "You will see a far greater percentage of blacks than whites in the range of IQs 75 or below, at which point we are close to the typical boundary for mild mental retardation."
Sabisky decided to resign even though Johnson's spokesman earlier had refused to condemn his views.
As well as the comments about black Americans, the aide, 27, also posted on Twitter that women's sport is "more comparable to the Paralympics than it is to men's," the Mail on Sunday reported, and in a 2016 interview with Schools Week magazine advocated eugenics - the practice of selective breeding.
Opposition politicians had called for Sabisky to be fired. He was appointed as part of a drive by Johnson's most powerful aide, Dominic Cummings, to hire mavericks, free thinkers and reformers to shake up the way the UK Government works from the inside.
Sabisky's exit marks another dramatic development in Johnson's Administration. Last week, Chancellor of the Exchequer Sajid Javid quit after Johnson made a grab to take full control over the Treasury.
It's not the first time Johnson's officials have had to deal with allegations of racism. Johnson himself has been criticised for articles he wrote for the Daily Telegraph newspaper, in which he used a racist term to refer to black people, and said Muslim women wearing the burqa looked like "letter boxes."