Downing Street adviser Andrew Sabisky has resigned after a string of his controversial remarks were uncovered.
Number 10 was already facing calls to sack him over previous comments on eugenics, mind-enhancing drugs for children, FGM and women's sport, but stayed silent.
Tory backbenchers demanded that he be dropped after it was also revealed he said MPs should pay attention to the "debate" around "very real racial differences in intelligence".
Mr Sabisky - who was hired in Prime Minister Boris Johnson's special adviser Dominic Cummings' appeal for "misfits and weirdos" - confirmed he quit on Monday night.
"The media hysteria about my old stuff online is mad but I wanted to help HMG [Her Majesty's Government] not be a distraction," the 27-year-old tweeted.
"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."
He also claimed to be "in the middle of a giant character assassination".
One of the posts that came under scrutiny was a comment posted beneath a blog in January 2014 made by a user named "Andrew Sabisky" and using the same avatar as his Twitter account.
"There are excellent reasons to think the very real racial differences in intelligence are significantly - even mostly - genetic in origin, though the degree is of course a very serious subject of scholarly debate," it said.
Downing Street was criticised for refusing to say whether or not Mr Johnson agreed with Mr Sabisky's views, prompting fury from some Conservatives.
Former minister Caroline Nokes said there "must be no place in government for the views he's expressed".
Select committee chair William Wagg declared "there is no way to defend it" and added: "I cannot be the only one uncomfortable with recent Number 10 trends."
One Tory staffer told Sky News after the resignation was confirmed: "I'm glad it's happened but he should never have been hired in the first place. Two days of bad headlines for nothing."
Ian Lavery MP, chair of the Labour Party, said Mr Sabvisky should "never have been appointed in the first place".
He added: "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."
Pregnancies, drugs, Muslims and FGM - Andrew Sabisky's alleged past comments:
"There are excellent reasons to think the very real racial differences in intelligence are significantly - even mostly - genetic in origin, though the degree is of course a very serious subject of scholarly debate." Online comment from January 2014.
"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" A comment on Dominic Cummings' blog from August 2014.
"From a societal perspective the benefits of giving everyone modafinil once a week are probably worth a dead kid once a year." A Schools Week interview in July 2016.
"Eugenics are about selecting 'for' good things... Intelligence is largely inherited and it correlates with better outcomes: physical health, income, lower mental illness. There is no downside to having IQ except short-sightedness." The same Schools Week interview, July 2016.
"Will institutionalized power-sharing (as in Northern Ireland) become the norm in the West - not between Catholic and Protestant, but between Muslim and non-Muslim (by around 2050 Britain is forecast to be a majority Islamic nation on current birthrate trends)?" As part of a book review from February 2014.
"...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." An online comment from January 2014.
"It is still unclear to what extent FGM represents a serious risk to young girls, raised in the UK, of certain minority group origins. Much of the hue and cry looks more like a moral panic." An online comment from July 2014.
"Individual differences in intelligence and academic outcomes are largely a result of genes, not environments." A paper published online.
"I am always straight up in saying that women's sport is more comparable to the Paralympics than it is to men's" A Twitter post from May last year.