Ian Blackford was told to withdraw the remark, but refused – sparking uproar from Conservative MPs opposite.
“He described African people as having watermelon smiles and other disgusting smears. If that’s not racist, I don’t know what is,” he said.
“Does the prime minister honestly believe that this man is fit for the office of prime minister?”
Ms May ducked the accusation, telling Mr Blackford he should be asking her about own actions in Downing Street, rather than making accusations about Mr Johnson.
The attack came after the former foreign secretary refused to apologise for his notorious newspaper column likening women in burqas to “letterboxes”, during Tuesday’s TV debate.
Instead, he said: “Of course, in so far as my words have given offence, over the last 20 or 30 years that I've been a journalist, people have taken those words out of my article and escalated them. Of course I'm sorry for the offence that they have caused.”
Mr Johnson then he referred to his own Muslim great-grandfather, who he said came to Britain because it was “a beacon of generosity and openness”.
After prime minister’s questions, one Tory backbencher, Bill Wiggin, condemned Mr Blackford’s attack and urged the Speaker to “ensure that words like racist are not common parlance in this House”.
But Mr Bercow said the word racist was “not in itself unparliamentary” – arguing he had to “judge context and to make an assessment of what is seemly in the chamber”.
Mr Blackford began his comments by saying: ‘Does the prime minister agree with the front runner set to succeed her that the Scottish people are a verminous race that should be placed in ghettos and exterminated?”
And he went on: “Words matter and actions matter. The man who published those words in his magazine, the prime minister thought was fit for the office of a top diplomat and he hasn't stopped there.
“He said that Scots should be banned from being prime minister and that a pound spent in Croydon was worth more than a pound spent in Strathclyde. This is a man who is not fit for office.”
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