Number 10 has refused to condemn past remarks on pregnancies, eugenics and race reportedly made by a new adviser.
Downing Street is under increasing pressure to sack Andrew Sabisky – appointed after the PM’s chief adviser Dominic Cummings called for “misfits and weirdos” to apply for jobs there.
Labour said the lack of condemnation of the “appalling” remarks was “disgusting”.
Mr Sabisky has been contacted by the BBC for comment.
When asked on Monday, Downing Street would not confirm his appointment or what type of role he may have taken on.
Boris Johnson’s spokesman added: “The prime minister’s views on a range of subjects are well publicised and documented.”
In response, Conservative MP Caroline Nokes tweeted: “Cannot believe No 10 has refused to comment on Andrew Sabisky. I don’t know him from a bar of soap, but don’t think we’d get on… must be no place in government for the views he’s expressed.”
In a comment on a 2014 blog post on Mr Cummings’ website, made by a user called “Andrew Sabisky” that uses the same picture as his Twitter page, he suggested that compulsory contraception could be used to stop a “permanent underclass”.
“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,” he wrote.
“Vaccination laws give it a precedent, I would argue.”
In a comment on another blog post on a different website in 2014, what appears to be the same user suggested black Americans had a lower average IQ than white Americans.
In a comment on a different blog that same year, a user with his name said: “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.”
Mr Sabisky also suggested to Schools Week in July 2016 that the benefits of a purported cognitive enhancer, which can prove fatal, are “probably worth a dead kid once a year”.
“Eugenics are about selecting ‘for’ good things,” he said in the same interview. “Intelligence is largely inherited and it correlates with better outcomes: physical health, income, lower mental illness.
And in a Twitter post from 2019, he said: “I am always straight up in saying that women’s sport is more comparable to the Paralympics than it is to men’s.”
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’s shadow Cabinet Office minister Jon Trickett said: “There are really no words to describe Boris Johnson’s appointment as one of his senior advisers a man who is on record as supporting the forced sterilisation of people he considers not worthy.
“He must of course be removed from this position immediately.”
And acting Liberal Democrat leader Ed Davey urged the prime minister to “put an end to the offence caused and sack Andrew Sabisky”.
“This Conservative government is a national embarrassment,” he said. “By giving Dominic Cummings such power and then failing to control him, Boris Johnson is revealing who really is in charge.”
When asked about the remarks on Sky News, Environment Secretary George Eustice said it was a “matter for Dominic Cummings and Number 10”.
On Sunday, Transport Secretary Grant Shapps told the BBC: “I don’t know the individual but they are particularly not views that I or the government shares in any way, shape or form.”