Downing Street has refused to condemn comments from an adviser who once suggested enforcing the uptake of contraception to stop unplanned pregnancies "creating a permanent underclass".
Boris Johnson is under mounting pressure to sack Andrew Sabisky, who was drafted in to Number 10 after the Prime Minister's chief adviser, Dominic Cummings, called for "misfits and weirdos" to apply to advise the Government.
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 also 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 that black Americans have a lower average IQ than white Americans.
Downing Street 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.
A Number 10 spokesman said: "I'm not going to be commenting on individual appointments."
The spokesman 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.
In a speech in 2013 he said any discussion about equality had to take account of the fact that 16% of "our species" had an IQ below 85 while around 2% 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 Mr Johnson was editor of the Spectator, the magazine carried an article from columnist Taki 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."
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."
Ms Sturgeon said: "These are really not acceptable headlines for any government to be generating (or allowing to be generated).
"They need to get a grip fast and demonstrate some basic but fundamental values in the terms of our public debate."
Geneticist and broadcaster Dr 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."
He 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."
It is understood that special advisers are prepared to boycott meetings where Mr Sabisky is present and refuse to reply to any emails he sends.