Conservative leadership frontrunner Boris Johnson is a "racist" and "not fit for office", the SNP's Westminster leader has said.

Ian Blackford accused the former London mayor of "stoking division" and of having "a record of dishonesty" in stormy scenes at Prime Minister's Questions on Wednesday.

His claim of racism sparked fury among MPs on the Conservative benches, who demanded he withdraw the remark, while Speaker John Bercow advised Blackford to "weigh his words".

But the SNP MP stood by his comments, raising past statements by the ex-foreign secretary in which he described Muslim women as "letterboxes" and black Africans as having "watermelon smiles".

Addressing outgoing Prime Minister Theresa May, Blackford began the exchange by raising a poem intended as satire Johnson published when he was a journalist in 2004.

He asked May: "Does the Prime Minister agree with the frontrunner set to succeed her that the Scottish people are a verminous race that should be placed in ghettos and exterminated?"

The Prime Minister said her party welcomes "the contribution from people of every part of this UK... and long may Scotland remain part of it".

Blackford went on: "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."

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

As Tory MPs objected loudly to the remark, the Speaker intervened to suggest Blackford "withdraw any allegation of racism against any particular member".

John Bercow added: "I don't think that this is the forum and I don't think it's the right way to behave."

The SNP's Westminster leader 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 answered: "Any Conservative prime minister in the future will be better for Scotland than the SNP."

Later, Conservative MP Bill Wiggin raised a point of order to highlight his concerns over the use of the word "racist" aimed at Johnson.

The North Herefordshire MP told 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'...

"Please will you do all you can to make sure that words like 'racist' are not common parlance in this House?"

Bercow responded: "The word in question is not of itself unparliamentary. The issue is to judge context and to make an assessment of what is seemly in the chamber.

"I made my own assessment and I advised the House accordingly, and (Blackford).

"It was only when I heard the full flow of the words that I was able to make an assessment, and I think it would be wise for colleagues to bear in mind the general principle that one does not impute dishonour to another member."