The SNP’s leader at Westminster leader has branded Boris Johnson a “racist” who is unfit to lead the country.
Speaking during prime minister’s questions, Ian Blackford highlighted previous comments Mr Johnson had made about Muslims, Africans and Scots.
He refused to withdraw his allegation of racism despite being urged to do so by Speaker John Bercow.
Prime Minister Theresa May responded by saying whoever succeeds her “will be better for Scotland than the SNP”.
Mr Johnson, the former foreign secretary and London mayor, is widely seen as the favourite to win the contest to become the next Conservative leader – and therefore the next prime minister.
He secured a commanding lead in the latest round of voting on Tuesday, winning the support of 126 Tory MPs – 80 more than the second-placed Jeremy Hunt.
Nicola Sturgeon, the SNP leader and Scotland’s first minister, has repeatedly predicted that Mr Johnson would be a “disaster” in 10 Downing Street and would boost the case for independence.
Mr Blackford used his weekly question to the prime minister to claim that Mr Johnson had called for Scottish people to be “banned” from being prime minister.
He was apparently referring to an article written in the Spectator magazine by Mr Johnson in 2005.
Mr Blackford went on to say that Mr Johnson is “not only racist” but is also “stoking division in communities and has a record of dishonesty”.
His remarks caused uproar in the Commons chamber, with Mr Bercow urging Mr Blackford to be “extremely careful in the language he uses”.
Mr Bercow said: “I think it would be much better for now were he to withdraw any allegation of racism against any particular member.
“I don’t think this is the forum and I don’t think it’s the right way to behave.”
Analysis by Laura Kuennsberg, BBC political editor
The truth is Boris Johnson, alongside being a politician, has been a columnist who has made a coin by being provocative.
It was his job to be so, but he has offended a lot of people in the process.
Of course, some of his comments have had real life consequences too – such as the case of Nazanin Zaghari-Ratcliffe.
And it’s because of this that many Tory Party members, who do love the cheerleader aspect of Boris Johnson, also have grave reservations about him.
Mr Blackford stood by his claim, adding: “The member has called Muslim women letterboxes, he 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 prime minister honestly believe that this man is fit for the office of prime minister?”
The prime minister reminded Mr Blackford that the purpose of prime minister’s questions was to “ask the prime minister about the actions of the government”.
She went on to insist: “I believe any Conservative prime minister in the future will be better for Scotland than the Scottish National Party”.
Mrs May also said her 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”.