IAN Blackford sparked outrage in the Commons after doubling down on an accusation that Boris Johnson is “racist” at a fiery Prime Minister’s Questions.

The SNP’s Westminster leader also asked whether Theresa May agrees with the Tory leadership frontrunner that Scots are a “verminous race who should be placed in ghettoes and exterminated” – comments made in a Spectator column approved by former editor Johnson.

READ MORE: Fact-check: Did Boris Johnson call Scottish people a 'verminous race'?

May insisted that only Conservatives can protect the Union before the SNP Westminster leader added: "He [Johnson] said Scots should be banned from being prime minister ... this is a man who is not fit for office. The ultimate measure of a person is not how they stand in a moment of comfort but in times of challenge and this is a time of challenge.”

"Not only is the man racist..."

Blackford was then interrupted by the Speaker and cries from the Tory benches.

John Bercow said that Johnson should have been warned of the accusation and advised the SNP MP to “weigh his words”.

Blackford replied: "The member has called Muslim women 'letterboxes', described African people as having 'watermelon smiles' and another disgusting slur that I would never dignify by repeating. If that's not racist Mr Speaker I don't know what is."

Turning to May, he asked: "Does the prime minister honestly believe that this man is fit for the office of prime minister?"

May said that PMQs was about asking the prime minister questions about the actions of the government.

She added: "I can say that I believe any Conservative PM would be better for Scotland than the SNP."