Boris Johnson has been told to “immediately” sack a controversial new adviser as it emerged they had claimed black people were statistically more likely to be closer to the “boundary for mild mental retardation”. The Prime Minister was on Sunday facing an internal revolt over the hiring of Andrew Sabisky, a researcher and self-proclaimed “super forecaster”, with a number of ministerial advisers expressing their reluctance to work with him. It comes after a number of controversial remarks made by Mr Sabisky, a contractor brought in as part of Dominic Cummings’ recruitment drive for “weirdos and misfits”, were unearthed in recent days.