Conservative Rob Halfon, a former Skills Minister who chairs the Commons Education Committee, said the appointment had “gone badly wrong”.

Citing “some quite dark articles” Mr Young wrote “where he talks about the disabled, the working classes … and ‘progressive eugenics’”, he added: “I find this incredibly dark and very dangerous stuff.”

Mr Johnson admitted some of Mr Young’s tweets and remarks were “obnoxious”, “wrong”, “repellent”, “offensive”, “foolish” and “objectionable”.