The warning signs may have been there last month when Dominic Cummings, the influential and iconoclastic aide to Prime Minister Boris Johnson of Britain, invited “weirdos and misfits” to apply to work at Downing Street.

On Monday, one new recruit was at the centre of a furore over his past assertions that blacks have a lower IQ than whites, and that enforced contraception could prevent “creating a permanent underclass”.

As the Opposition Labour Party demanded the dismissal of the adviser, Andrew Sabisky, Downing Street declined all comment. It even managed to deepen the dispute by refusing to say that Johnson rejects eugenics, the widely dismissed field whose proponents advocate discouraging reproduction by people deemed to have undesirable genetic traits.

Then late on Monday, Sabisky himself announced that he was stepping down, complaining of “media hysteria”.

The episode has both shone a light on Johnson’s leadership team and illustrated the sway that Cummings holds in Downing Street, where he cuts a distinctive, scruffy figure and has established himself as one of the most polarising figures in government.

The decision to hire Sabisky, may even so prove damaging for Johnson, in part because he has himself used racist language in his writing.

Johnson is already facing questions over who paid for a £15,000 winter vacation taken by the Prime Minister and his partner on the island of Mustique in December and early January.

It was as he basked there in the Caribbean sunshine that Cummings, one of the architects of the 2016 Brexit campaign, issued his appeal on his blog for “super-talented weirdos” to come and work alongside him at Downing Street. The idea was to shake up a cadre of top officials that Cummings considers complacent.

“We need some true wild cards, artists, people who never went to university and fought their way out of an appalling hell hole, ” Cummings wrote in a post that prompted applications from, among others, Uri Geller, the self-styled magician-psychic famous for bending spoons.

Geller seems not to have got a callback. But Sabisky, who describes himself on Twitter as a “researcher” and “super-forecaster”, was hired as an adviser in the Prime Minister’s office. Then, over the weekend, reports began to surface about his various online postings. On Cummings’s blog in 2014, for example, Sabisky suggested forced “universal uptake of long-term contraception at the onset of puberty” for what he called the “underclass.”