Jeremy Hunt has said he agrees "150%" with US President Donald Trump's assessment of London Mayor Sadiq Khan's record on tackling knife crime.
Mr Trump has been involved in a long-running feud with the mayor, who accused the president of being a "poster boy for racists".
The latest salvo in their transatlantic exchange came after Mr Trump dubbed Mr Khan a "disaster" on Twitter when retweeting a post from political commentator Katie Hopkins over the weekend.
He also called him a "national disgrace who is destroying the city of London".
The young people of London are amazing.
We need you to know how important you are, that your lives matter & that London would not be London without you.
That's why you shouldn't carry a knife.
— Sadiq Khan (@SadiqKhan) June 17, 2019
Foreign Secretary Mr Hunt intervened in the row during a Tory leadership hustings in Westminster, saying that the US president "has his own style" and he "would not use those words myself".
"But the sentiment is enormous disappointment that we have a Mayor of London who has completely failed to tackle knife crime and has spent more time on politics than the actual business of making Londoners safer and in that I 150% agree with the president."
But Tory leadership rival and Home Secretary Sajid Javid said it was "unbecoming" for a president to intervene in an ally's domestic politics.
He said: "I think President Trump should stick to domestic policies and I think it is unbecoming of a leader of such a great state to keep trying to interfere in other countries' domestic policies."
He added: "The president is right to be concerned about serious violence but he should be concerned about the serious violence in his own country where it is more than 10 times higher than it is in the UK."
LONDON needs a new mayor ASAP. Khan is a disaster – will only get worse! https://t.co/n7qKI3BbD2
— Donald J. Trump (@realDonaldTrump) June 15, 2019
The original post by Hopkins called the capital "Stab-City" and "Khan's Londonistan" alongside two screenshots of BBC News articles detailing the violence.
Speaking in central London on Monday, Mr Khan said: "It's for Donald Trump to explain his tweets, not for me, but it's remarkable that you've got the president of the USA amplifying the tweets of a far-right activist, amplifying a racist tweet.
"That's one of my concerns about Donald Trump – he's now seen as a poster boy for racists around the world, whether you're a racist in this country, whether you're a racist in Hungary, a racist in Italy, or a racist in France.
"He's now a poster boy for the far-right movement and that should cause us huge concern."
The mayor said Mr Trump was "obsessed" with him and said that many cities were facing an increase in violent crime.
He added: "There are many good leaders in America facing massive increases in violent crime, they have my support to make sure we learn lessons from each other and that we work together to grapple the issue of violent crime taking place in many cities across the Western world."