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."
Mr Johnson is currently odds-on favourite to be elected the next Prime Minister, after he secured the votes of 114 MPs at the first round of voting in the contest to replace Theresa May.
However, he has been criticised in some sections of his party and beyond for avoiding scrutiny after refusing to confirm he will attend a serious of televised debates between the main leadership contenders.
Broadcaster Channel 4 is believed to be preparing to 'empty chair' the former Foreign Secretary, with rival candidate Rory Stewart telling the Daily Mirror: "It would be terrible if he (Johnson) didn’t take part.
"I want to go up against him.
"We deserve to know - is he going to prorogue parliament?"