Boris Johnson says Trump's tweets telling congresswomen to "go back" where they came from were "totally unacceptable"
"If you are 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."
However, Johnson and his rival to become the UK's next prime minister resisted calling Trump racist.
Johnson has also come under fire for his own past offensive comments.
Donald Trump's tweets telling progressive congresswomen to "go back" where they came from were "totally unacceptable," the favourite to replace Theresa May as the United Kingdom's prime minister, has said.
Boris Johnson said it was not appropriate to "use that kind of language" in public life.
"If you are 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," he told a hustings event organised by the Sun Newspaper.
"That went out decades and decades ago and thank heavens for that, so it's totally unacceptable."
Johnson echoed prime minister May, whose spokesman also condemned the tweets earlier on Monday.
"The prime minister's view is that the language used to refer to these women was completely unacceptable," the spokesman said.
They were also endorsed by his rival for Conservative party leader, the Foreign Secretary, Jeremy Hunt, who said he would be "appalled" if such comments were made about his own family.
"I have three half-Chinese children, and they are British citizens born on the NHS, and if anyone ever said to them 'go back to China' I would be utterly appalled," he told the event.
However, both men refused to label Trump's comments as racist.
"This is a president of a country which happens to be our closest ally and so it is not going to help the situation to use that kind of language about the president," Hunt said.
Trump tweeted on Sunday morning asking the "'Progressive' Democrat Congresswomen" — widely interpreted as a reference to Reps. Alexandria Ocasio-Cortez of New York, Rashida Tlaib of Michigan, Ayanna Pressley of Massachusetts, and Ilhan Omar of Minnesota — to "go back" to their "corrupt" and "broken and crime infested" countries.
Trump on Monday doubled down on his comments, saying that many people agree with him about the congresswomen.
"If you're not happy in the US, if you're complaining all the time, very simply: You can leave. You can leave right now," Trump said.
He added that if they did leave the US there would be "many people that won't miss them."
There is growing tension between Washington and London after the publishing last week of leaked private memos in which the UK's ambassador to the US, Kim Darroch, criticized the Trump administration.
In the notes, the UK ambassador to the US said Trump's government was "uniquely dysfunctional" and added: "We don't really believe this administration is going to become substantially more normal."
He also described Trump's presidency as "dysfunctional," "incompetent," and "inept."
Darroch quit after Trump described him as a "very stupid guy" and a "pompous fool" and said he would not work with him in the future.
Johnson was later accused by colleagues of having thrown Darroch "under a bus" by refusing to publicly back him remaining in post.
Johnson also under fire for offensive remarks
Aurelien Meunier/Getty Images
Johnson also came under pressure on Monday over his own history of making offensive comments about ethnic minorities and gay people.
As a newspaper and magazine columnist, Johnson wrote a piece referring to black Africans as "piccaninnies" with "watermelon smiles."
Last year the Conservative party launched an internal investigation after he compared Muslim women who wear the burqa to "letterboxes" and "bank robbers."
Asked on Monday about his previous comments, as revealed by Business Insider, referring to gay men as "tank-topped bumboys" he told the Sun debate that his remarks had been "twisted" and insisted that he was strongly supportive of equal rights for gay people.
Last week Johnson insisted his previous comments about ethnic minorities were "wholly satirical".