The chairman of the Conservative Muslim Forum has compared Boris Johnson to Adolf Hitler and said he will end his 36-year membership of the party if he becomes prime minister.
Mohammed Amin said that Mr Johnson, who won the first round of the Tory leadership race with nearly three times the votes of his closest rival, was not "sufficiently moral" to lead the country.
He said the former foreign secretary would have been aware of the knock-on effects of denigrating Islamic dress in an article for the Daily Telegraph last summer, in which he compared women wearing religious headgear to bank robbers and letterboxes.
"Mr Johnson is a very intelligent man and when he wrote his article ... he knew exactly what effect it would have," Mr Amin told BBC Radio 4's Today programme.
"It would lead to Muslim women, who wear niqab and burqa, being verbally abused on the streets, in certain cases being physically assaulted with people trying to tear it off.
"I'm sure it was clear to him that was the likely consequence of his article.
"He chose to mock Muslim women who wear niqab and burqa for his own purposes."
Mr Amin, who has been a Tory member for 36 years, added: "We don't expect our leaders and prime ministers to be saints, but we do require a basic level of morality and integrity, and of all the candidates in the leadership election, Boris Johnson is the only one that I believe fails that test.
"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 compared Mr Johnson's popularity to that of the Nazi leader, saying: "A lot of Germans thought that Hitler was the right man for them.
"I'm not suggesting he wants to send people to the gas chambers, 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."
Former cabinet minister Priti Patel defended Mr Johnson's record, saying his comments had been misinterpreted as he was a champion of women's rights and girls education around the wolrd.
Ms Patel, who is backing Mr Johnson's campaign, said: "I'm sorry to hear that a member of the party feels that way.
"I recall that article. It was a defence of women's right to wear whatever they like and I think it is fair to say, Boris himself has said this, the language that he used, he has apologised for any offence that has been caused.
"I think fundamentally he was not mocking women."
The row came as contenders to replace Theresa May were vying to claim the mantle of the “stop Boris” candidate, after his emphatic victory in the first round of voting.
His nearest rival Jeremy Hunt, the foreign secretary, was in second place with 43 votes to Mr Johnson's 114, with Michael Gove, the environment secretary, in third place on 37.
Meanwhile, Matt Hancock is understood to be considering pulling out of the race to support another candidate with a better chance of winning the 33 votes needed to get past the next round.
The health secretary reportedly met Sajid Javid, the home secretary, but the meeting appears not to have resulted in any agreement.
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