Donald Trump has told Prime Minister Theresa May to focus on "terrorism" in the UK after she criticised his sharing of far-right videos.
"Don't focus on me, focus on the destructive Radical Islamic Terrorism that is taking place within the United Kingdom," Mr Trump tweeted.
The US president had earlier retweeted three inflammatory videos posted online by a British far-right group.
Mrs May's spokesman said it was "wrong for the president to have done this".
The US and the UK are close allies and often described as having a "special relationship". Theresa May was the first foreign leader to visit the Trump White House.
The speaker of the House of Commons has granted a request for an urgent question on the matter from Labour MP Stephen Doughty.
MPs have been reacting to the tweet, with Work and Pensions Secretary David Gauke backing Mrs May and calling Britain First a "ghastly, obnoxious organisation".
But while Education Secretary Justine Greening said she disagreed with Mr Trump's actions, they should not be allowed to damage the special relationship between the two countries.
The videos shared by Mr Trump, who has more than 40 million followers, were initially posted by Jayda Fransen, the deputy leader of Britain First, a group founded by former members of the far-right British National Party (BNP).
Ms Fransen, 31, has been charged in the UK with using "threatening, abusive or insulting words or behaviour" over speeches she made at a rally in Belfast.
Several leading UK politicians have criticised the president for retweeting her posts, as has the Archbishop of Canterbury Justin Welby, who said it was "deeply disturbing" that Mr Trump had "chosen to amplify the voice of far-right extremists".
And it has led to renewed calls for Mr Trump's planned state visit to the UK to be cancelled, although Downing Street said on Wednesday that the invitation still stood.
London Mayor Sadiq Khan, who has previously called for the "ill-judged" trip to be cancelled, said: "It beggars belief that the president of our closest ally doesn't see that his support of this extremist group actively undermines the values of tolerance and diversity that makes Britain so great.
"After this latest incident, it is increasingly clear that any official visit at all from President Trump to Britain would not be welcomed."
In hitting out at Mrs May, Mr Trump first tagged the wrong Twitter account, sending his statement to a different user with just six followers. He then deleted the tweet and posted it again, this time directing the message to the UK PM's official account.
After already condemning Mr Trump's actions on Wednesday, Brendan Cox - whose wife, MP Jo Cox, was murdered by a right-wing extremist who shouted "Britain first" before committing the act - told the US president to focus on problems in his own country.
TV presenter and journalist Piers Morgan, who has supported Mr Trump in the past, said the president "owes our prime minister an apology, not a lecture" after he "publicly endorsed the most extreme bunch of Islamophobe fascists in Britain".
But American conservative commentator Ann Coulter - who is one of the 45 people followed on Twitter by Mr Trump and retweeted the videos first - defended her president's words to Mrs May, saying he had "only given as good as he gets".