London (CNN) Boris Johnson, the Conservative Party leadership hopeful who is widely expected to become Britain's Prime Minister next week, has described US President Donald Trump's racist tweets attacking four congresswoman as "totally unacceptable."

Asked about the President's comments in a debate on Monday against his challenger for the post, Jeremy Hunt, Johnson said he "can't understand" how an American leader would use such language -- but stopped short of calling the attack racist.

Trump told four nonwhite Democratic congresswomen on Sunday that they should "go back and help fix the totally broken and crime infested places from which they came," even though three of the four were born in the US and the fourth is a naturalized citizen.

In response, the four lawmakers made clear in a Monday news conference that they're not going anywhere. "I want to tell children across this country is that no matter what the President says, this country belongs to you," said Rep. Alexandria Ocasio-Cortez of New York. "And it belongs to everyone and today that notion, that very notion was challenged."

"If you're the leader of a great multiracial, multicultural society you simply cannot use that kind of language about sending people back to where they came from," Johnson said of Trump's tweets during the UK leadership debate.

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