An adviser in British prime minister Boris Johnson's office resigned on Monday (17) after online comments resurfaced where he linked intellect to race and seemed to advocate a eugenics policy.
Andrew Sabisky once suggested that black Americans had a lower average intelligence than white Americans.
He also said in a 2014 web post that one way to stop unplanned pregnancies "creating a permanent underclass" was to force people to use contraception.
Sabisky said he was in the middle of a "giant character assassination" and was stepping down because he did not want to be a distraction.
"The media hysteria about my old stuff online is mad but I wanted to help HMG, not be a distraction," he said on Twitter using the initials for Her Majesty's Government.
"Accordingly I've decided to resign as a contractor."
He said he hoped the government hired more people with "good geopolitical forecasting track records and that media learn to stop selective quoting".
"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," he said, adding that he had "a lot of other things to do" with his life.
Downing Street had repeatedly refused to say whether Johnson supported the views expressed by Sabisky.
"I'm not going to be commenting on individual appointments," a spokesman said.
The main opposition Labour Party had called for his sacking.
"It's right that Andrew Sabisky is no longer working in government. He should never have been appointed in the first place," said party chairman Ian Lavery adding that Johnson had "serious questions to answer".
Johnson's chief adviser is Dominic Cummings, 48, who masterminded the official campaign for Britain to leave the European Union.
Writing on Cummings's website in 2014, Sabisky 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."
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