As U.K. Foreign Secretary Boris Johnson burnishes his euro-skeptic credentials for a possible Conservative Party leadership bid, secret polling has emerged showing he was unpopular with voters and had to be kept out of the public eye during this year's election campaign, a former government aide has said.

Nick Timothy said the Tories focused their election campaign on Theresa May because their private research showed "nobody in the Cabinet is popular — keep them away, even Boris." Timothy resigned as May's co-chief of staff after the results. Going into the election, May was more popular than any of her colleagues individually and the Conservative's brand overall, the research showed.

Timothy's comments, in an interview for a forthcoming book on the June vote, emerged after Johnson, the former mayor of London, launched what some observers regarded as a direct attempt to undermine the prime minister. Johnson went out on a limb on Saturday with a 4,000-word essay about his "glorious" vision for post-Brexit Britain, in the Tory-supporting Daily Telegraph newspaper.