The chairman of the Conservative Muslim Forum has made a comparison between Boris Johnson and Adolf Hitler.
Mohammed Amin said he would quit the Tory Party after many years as a member if the former foreign secretary was elected leader.
Mr Amin told BBC Radio 4’s Today programme: “I am not prepared to be a member of a party that chooses him as its leader.
“I would resign after 36 years.”
Asked about Mr Johnson’s popularity with grassroots members of the party, Mr Amin said: “There are many horrible people who have been popular. Popularity is not the test.
“The test is, is this person sufficiently moral to be prime minister, and I believe he fails that test.”
Mr Amin added: “A lot of Germans thought that Hitler was the right man for them.”
Told that that was a shocking comparison, Mr Amin said: “Yes.
“I am not saying Boris Johnson wants to send people to the gas chamber, clearly he doesn’t.
“He’s a buffoon.
“But he, as far as I’m concerned, has insufficient concern about the nature of truth for me to ever be a member of a party that he leads.”
Mr Amin went on: “We don’t expect our politicians, our prime ministers, to be saints.
“But we do require a basic level of morality and integrity.
“And of all the candidates in the Conservative Party leadership election, Boris Johnson is the only one that I believe fails that test.”
Mr Amin said a column Mr Johnson wrote last August comparing women who wore burkas to “letter boxes and bank robbers” had put some Muslim women at risk.
“He knew exactly what effect it would have – it would lead to Muslim women who wear niqab and burka being verbally abused on the streets; in certain cases being physically assaulted, with people trying to tear it off.
“He chose to mock Muslim women who wear niqab and burka for his own purposes.”
Former Cabinet minister Priti Patel, who exited the cabinet after conducting unsanctioned meetings with Israeli officials, backed Mr Johnson, telling the BBC: “That article was Boris’s very clear defence of women’s rights to wear whatever they like, and it was not written to mock in the way in which was asserted.”
Meanwhile, Foreign Secretary Jeremy Hunt accused Mr Johnson of hiding from the media and preventing a wider debate in the Tory Party.