Boris Johnson’s spokesman has refused to say whether the prime minister thinks black people have lower IQs on average, or agrees with eugenics, after No 10 hired an adviser with highly controversial views.

In a tense briefing with the media, the prime minister’s deputy official spokesman declined several times to distance Johnson from the views of his adviser, Andrew Sabisky, who has suggested “enforced contraception” be used to prevent the creation of a “permanent underclass”.

Labour has called on No 10 to sack Sabisky, who is believed to be contracted by Downing Street under Johnson’s de facto chief of staff, Dominic Cummings, to work on special projects.

Johnson’s official spokesman refused to comment on Sabisky, his controversial views, or whether the prime minister agreed with them.

“The prime minister’s views are well publicised and well documented,” the spokesman said more than 10 times, when asked to give Johnson’s views on the intelligence of black people and eugenics, the study of methods to selectively breed people to improve the human race.

Sabisky, 27, has claimed black Americans have a lower than average IQ than white people and are more likely to have an “intellectual disability”. He also tweeted: “I am always straight up in saying that women’s sport is more comparable to the Paralympics than it is to men’s.”

In an interview from 2016, Sabisky said he was interested in the narcolepsy drug modafinil, which also reduces the need for sleep in healthy people by two-thirds and potentially helps brain function, although there is evidence of a higher risk of people getting Stevens-Johnson syndrome, a life-threatening skin condition.

Sabisky said: “From a societal perspective the benefits of giving everyone modafinil once a week are probably worth a dead kid once a year.”

He wrote on Cummings’ website in 2014 that in order to get around unplanned pregnancies in the UK, long-term contraception should be legally enforced.

“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.”

Ian Lavery, the Labour party chairman, said: “It is disgusting that not only has No 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.”

Scotland’s first minister, Nicola Sturgeon, tweeted: “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.”

The geneticist Dr Adam Rutherford also criticised the comments. He tweeted: “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.”

Grant Shapps, the transport secretary, said over the weekend that Sabisky’s comments were “not my views and those are not the views of the government”.

However, the prime minister’s deputy official spokesman said Shapps was speaking only for himself when he made that statement.