The spokesperson of the outgoing British Prime Minister Theresa May said on Monday that U.S. President Donald Trump's remarks on several minority Democratic lawmakers telling them to "go back" to the "places from which they came" are "completely unacceptable".

But the spokesperson declined to give further remarks on the issue.

The move came one day after Trump ignited a Twitter war with several minority Democratic lawmakers.

"Why don't they go back and help fix the totally broken and crime-infested places from which they came," Trump said in a series of tweets targeting at four Democratic lawmakers who were ethnic minorities.

Three of the four were born in the United States and one moved to the United States as a child refugee.

The group of four lawmakers, sometimes known as "the squad" among peers, was often attacked by Trump for their progressive positions.