The leader of the U.K.'s Scottish National Party called Boris Johnson, the top contender in the Conservative Party's race for a new head —who will also be prime minister — a racist and refused to withdraw his allegations on Thursday when pressed on the comments.

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"The member has called Muslim women letter boxes, described African people as having watermelon smiles and another disgusting slur I would never dignify by repeating. If that's not racist, Mr. Speaker, I don't know what is," Ian Blackford said during prime minister's questions.

Blackford's remarks were reported by BBC and shared in a video by British ITV News reporter Shehab Khan on Twitter.

WATCH: Remarkable moment during #PMQs as Ian Blackford calls Boris Johnson racist and then when asked to withdraw the comment by the Speaker, just doubles down. pic.twitter.com/Gh3UTm0kdK — Shehab Khan (@ShehabKhan) June 19, 2019

Blackford said Johnson should be "banned from being Prime Minister," claiming the member of parliament and former mayor of London is "unfit for office."

"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," Blackford said.

"This is a time of challenge, and so I ask, does the prime minister realize not only is the member racist, he is stoking divide in communities and has a record of dishonesty," Blackford continued, before being stopped by Speaker John Bercow who called for order.

Bercow asked Blackford to be "careful in the language he chooses," and asked Blackford to withdraw any allegations of racism against another member. Blackford would not, instead doubling down and issuing examples of Johnson's alleged racist comments.

Prime Minister Theresa May Theresa Mary MayAre US-Japan relations on the rocks? Trump insulted UK's May, called Germany's Merkel 'stupid' in calls: report Bolton says Boris Johnson is 'playing Trump like a fiddle' MORE responded that the purpose of prime minster's questions was to ask about "the actions of the government," according to BBC.

She also said she believes "any Conservative prime minister in the future will be better for Scotland than the Scottish National Party," BBC reports.

The Conservative Party is looking for its next leader after May announced last month she'd be resigning amid failed Brexit negotiations.

The race is down to three contenders, with Johnson leading in the latest vote with 157 votes from members of Parliament, according to BBC.

The remaining two will compete in a run-off and a winner will be announced in July, according to BBC.