Downing Street has said its adviser Andrew Sabisky has resigned following criticism of reported past remarks on pregnancies, eugenics and race.
Labour had called for Mr Sabisky to be sacked for suggesting black people had lower average IQs than white people.
He is also alleged to have said compulsory contraception could prevent “creating a permanent underclass”.
Mr Sabisky appears to have tweeted that he did not want to be “in the middle of a giant character assassination”.
“If I can’t do the work properly there’s no point, and I have a lot of other things to do with my life,” he added.