The SNP's Westminster leader has refused to withdraw allegations that Boris Johnson is "racist".
Ian Blackford listed a range of instances when the frontrunner for the Tory leadership used offensive language during a heated clash in Prime Minister's Questions.
These include referring to Muslim women wearing burkas as "letterboxes", people from Africa as having "watermelon smiles" and described Scots as a "verminous race".
He thundered: "This is a man who is not fit for office. It has been said Mr Speaker the ultimate measure of a person is not where they stand in moments of comfort but where they stand at times of challenge and controversy.
"This is a time of challenge and so I ask does the Prime Minister realise he is not only is the member racist, he is stoking divisions in communities and has a record of dishonesty."
At that point the Speaker interrupted and warned him that he needed to be "extremely careful" in his language.
John Bercow told Mr Blackford to "weigh his words" and advised him to "withdraw any allegation of racism against any particular measure".
But Mr Blackford doubled down saying: "But 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.
He added: "If that's not racist Mr Speaker I don't know what is."
Mr Blackford also referenced Mr Johnson's record of insulting Scotland.
As editor of the Spectator the Uxbridge MP published a poem describing Scottish people as “a verminous race who should be placed in ghettos and exterminated".
It went on to describe Scots as “tartan dwarves” who were “polluting our stock”.
Mr Blackford told MPs: "words matter and actions matter and the man who published those words in his magazine the prime minister thought was fit for the office of our top diplomat.
"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.
The SNP leader asked Theresa May if she thinks Boris Johnson is "fit for the office of Prime Minister".
Theresa May hit back at Mr Blackford suggesting he did not understand how Prime Minister's Questions works despite doing it "for a number of years".
"I believe any Conservative Prime Minister in the future will be better for Scotland than the Scottish Nationalist Party."
After Prime Minister's Questions Tory Bill Wiggin raised a point of order about Mr Blackford's question.
He told Speaker John Bercow: "Since you took the chair you have been a stalwart defender of backbenchers. You've also stood up to bad parliamentary behaviour, like the use of the word 'racism'.
"I am deeply upset that your chairmanship has been undermined dramatically because of the very calm and polite advice you gave to honourable members, leaders of political parties in this House, that was ignored.
"Please will you do all you can to make sure that words like 'racist' are not common parlance in this House?"
The Speaker confirmed that the term "racist" is not of itself unparliamentary but added that it would be "wise for colleagues to remember the general principle that one does not impute other members".
