An adviser to British Prime Minister Boris Johnson, who once said black Americans have a lower average IQ than white Americans and had discussed the benefits of forced contraception, has quit amid backlash over his comments.

Andrew Sabisky said on Monday he was resigning because he had become "a distraction".

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Earlier, Johnson's spokesman repeatedly refused to comment when asked about Sabisky, whose appointment drew widespread criticism after the Mail on Sunday newspaper reported statements made in his name online in 2014 and 2016.

Media reported that Sabisky was hired following an unusual appeal earlier this year from Johnson's senior adviser Dominic Cummings for "weirdos and misfits with odd skills" to help bring new ideas to the UK's government.

His resignation is a blow to that effort, which had attracted criticism from those who said Cummings was sidestepping normal government recruitment processes.

"The media hysteria about my old stuff online is mad but I wanted to help [the government] not be a distraction," Sabisky said on Twitter.

"Accordingly I've decided to resign as a contractor ... 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."

Sabisky, who has referred to himself as a "super forecaster", said he hoped Johnson's office will hire more people with "good geopolitical forecasting track records" and that the "media learn to stop selective quoting".

An account in Sabisky's name made the comments about black IQ in a reply to a 2014 blog post written by an American professor discussing education disabilities in the United States.

In 2016, replying to a blog post written by Cummings, an account in Sabisky's name 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."

UK gov't refuses to condemn comments

Both the opposition Labour Party and at least two of Johnson's own Conservatives had called for Sabisky to be fired.

"Andrew Sabisky's presence in no.10 is a poor reflection on the Govt and there is no way to defend it. He needs to go. 'Weirdos' and 'misfits' are all very well, but please can they not gratuitously cause offence," Conservative legislator William Wragg wrote on Twitter before Sabisky resigned.

Andrew Sabisky’s presence in no.10 is a poor reflection on the Govt and there is no way to defend it. He needs to go. ‘Weirdos’ and ‘misfits’ are all very well, but please can they not gratuitously cause offence.

I cannot be the only one uncomfortable with recent no.10 trends. — William Wragg MP (@William_Wragg) February 17, 2020

Earlier on Monday, a spokesman for Johnson declined to discuss Sabisky's role in 10 Downing Street, saying "I'm not going to be commenting on individual appointments."

Spokesman Jamie Davies also refused to say whether the prime minister agreed with Sabisky's views, which critics say amount to support for eugenics, the now-discredited movement to improve the human race through selective reproduction.

"The prime minister's views on a range of subjects are well publicised and documented," Davies said.

Johnson has his own record of offensive comments, however.

In a speech in 2013, he said any discussion about equality had to take account of the fact that 16 percent of "our species" had an IQ below 85 while around 2 percent had an IQ above 130, adding: "The harder you shake the pack, the easier it will be for some cornflakes to get to the top."

In 2000, while Johnson was editor of the Spectator, the magazine carried an article from columnist Taki Theodoracopulos which said: "On average, Orientals are slower to mature, less randy, less fertile, and have larger brains and higher IQ scores. Blacks are at the other pole, and whites fall somewhere in the middle, although closer to the Orientals than the blacks."

The prime minister has also previously described black people as "piccaninnies" with "watermelon smiles", and more recently referred to Muslim women who wear the burqa, a full-body veil that covers the face as well, as "letterboxes".

Labour Party chairman Ian Lavery said: "It is disgusting that not only has Number 10 failed to condemn Andrew Sabisky's appalling comments, but also seems to have endorsed the idea that white people are more intelligent than black people.

"Boris Johnson should have the backbone to make a statement in his own words on why he has made this appointment, whether he stands by it and his own views on the subject of eugenics."