Downing Street adviser Andrew Sabisky has announced his resignation amid criticism over controversial past comments, including once suggesting the enforced uptake of contraception to stop unplanned pregnancies which were "creating a permanent underclass".
Writing on Twitter, Mr Sabisky said he had taken the decision to resign because he did not want to be a "distraction" to the Government and wanted to do "real work" rather than be in the "middle of a giant character assassination".
"If I can't do the work properly there's no point."
In tweets sent from his since deleted account, Mr Sabisky said: "The media hysteria about my old stuff online is mad but I wanted to help HMG not be a distraction.
"Accordingly I've decided to resign as a contractor.
"I hope no.10 hires more ppl w/ good geopolitical forecasting track records & that media learn to stop selective quoting."
In another tweet he said: "I know this will disappoint a lot of ppl but I signed up to do real work, not be in the middle of a giant character assassination: if I can't do the work properly there's no point, & I have a lot of other things to do w/ my life."
ITV News Political Correspondent Daniel Hewitt gives his analysis of events:
Mr Sabisky's resignation came just hours after Downing Street refused to condemn his past comments.
Earlier on Monday, a Number 10 spokesperson said: "I'm not going to be commenting on individual appointments."
Labour said Number 10's refusal to condemn the remarks was "disgusting", while Scottish First Minister Nicola Sturgeon said the Government must "demonstrate some basic but fundamental values".
Mr Sabisky was drafted in after the Prime Minister's chief adviser, Dominic Cummings called for "misfits and weirdos" to apply for jobs in Government.
Mr Sabisky reportedly once suggested that the benefits of a purported cognitive enhancer, which can prove fatal, are “probably worth a dead kid once a year”.
Writing on Mr Cummings’ website in 2014, he said: “One way to get around the problems of unplanned pregnancies creating a permanent underclass would be to legally enforce universal uptake of long-term contraception at the onset of puberty.
“Vaccination laws give it a precedent, I would argue.”
He also suggested black Americans have a lower average IQ than white Americans.
Downing Street did not confirm or deny whether Mr Sabisky had been hired by Number 10, saying it would not comment on individual appointments.
It repeatedly refused to say whether Mr Johnson supported the views expressed by Mr Sabisky on eugenics - the selective breeding of humans - or the IQ of black people.
The spokesperson added: "The Prime Minister's views on a range of subjects are well publicised and documented."
Number 10 insiders insisted that Mr Johnson did not support eugenics, but the Prime Minister has courted controversy with his views on IQ in the past.
Reacting to the news of Mr Sabisky's departure, Labour Party chairman Ian Lavery said: "It's right that Andrew Sabisky is no longer working in Government.
"He should never have been appointed in the first place.
"After Number 10 publicly stood by him today, Boris Johnson has serious questions to answer about how this appointment was made and whether he agrees with his vile views."