IAN Blackford told MPs "racist" Tory leadership contender Boris Johnson is "not fit for office" during a fierce attack at Prime Minister’s Questions.

The SNP Westminster leader refused to backtrack on the comments in the House of Commons despite an intervention by the Speaker.

Beginning his criticism of the frontrunner in the Conservative leadership race, Blackford asked whether the Prime Minister agrees with comments made in a 2004 Spectator column, published while Johnson was editor, calling Scots a “verminous race who should be placed in ghettoes and exterminated”.

May ignored the question, preferring to stick to the Government line.

"The Conservative and Unionist Party takes the people of every part of this UK not only seriously, but we welcome the contribution from people of every part of this UK, because that is what makes the UK the great country it is and long may Scotland remain part of it,” she said.

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

Blackford then rose again to launch a more direct attack on Johnson and the Tories.

The National:

The SNP Westminster leader said: "Well of course 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.

“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.

"Does the Prime Minister realise not only is the member racist, he is stoking division in communities and has a record of dishonesty?"

Blackford's comments prompted Speaker John Bercow to intervene, urging him to "weigh his words" as Tory MPs shouted "withdraw".

WATCH: Ian Blackford lambasts 'racist' Boris Johnson at PMQs

Bercow added: "I think it would be much better if for now he would withdraw any allegation of racism against any particular member. I don't think that this is the forum and I don't think it's the right way to behave."

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. Does the minister honestly believe that this man is fit for the office of prime minister?"

May once again issued a blunt response.

"I can say to [him] that I believe any Conservative prime minister in the future will be better for Scotland than the SNP," she said.