The United Kingdom government has refused to condemn an adviser to the prime minister who once said that black Americans have a lower average IQ than white Americans and who suggested enforcing contraception to stop unplanned pregnancies from "creating a permanent underclass".

Prime Minister Boris Johnson was under mounting pressure on Monday to sack Andrew Sabisky, but a government spokesperson repeatedly failed to answer reporters who asked whether Johnson agreed with the adviser's views, saying: "The prime minister’s views are well publicised and well documented."

Later on Monday, Sabisky announced he was quitting because he had become "a distraction".

More:

The main opposition Labour Party condemned the refusal to criticise the remarks, while Scottish First Minister Nicola Sturgeon said the government must "get a grip fast and demonstrate some basic but fundamental values".

Sabisky was believed to have been contracted by the government under Johnson's controversial chief adviser, Dominic Cummings.

Writing on Cummings's 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."

Downing Street has refused to say whether Johnson supported the views expressed by Sabisky on eugenics - the selective breeding of humans - or the IQs of black people.

But the prime minister has courted controversy with his views on IQ in the past.

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 two 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 that 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 "burka", or full veil, 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."

Labour MP Marsha de Cordova, a disability rights campaigner, tweeted: "This what we have come to, and it is sickening."

It's one thing to deplore eugenics on ideological, political, moral grounds. It's quite another to conclude that it wouldn't work in practice. Of course it would. It works for cows, horses, pigs, dogs & roses. Why on earth wouldn’t it work for humans? Facts ignore ideology. — Richard Dawkins (@RichardDawkins) February 16, 2020

Geneticist and broadcaster Adam Rutherford also criticised the comments, writing on Twitter: "Like Cummings, he appears to be bewitched by science, without having made the effort to understand the areas he is invoking, nor its history."

Rutherford said the moral repugnance of the remarks was overwhelming, adding: "I am all for scientifically minded people advising government. In fact, I am all for scientists advising government. From this perspective, Sabisky, and indeed Cummings, look bewitched by science without doing the legwork.

"Instead, this resembles the marshalling of misunderstood or specious science into a political ideology. The history here is important because this process is exactly what happened at the birth of scientific racism and the birth of eugenics."