The Government's response to Donald Trump's cancellation of a visit to Britain descended into chaos as Boris Johnson accused Sadiq Khan of endangering relations with the United States.
Downing Street was unable to say whether the Foreign Secretary spoke for the Government when he said the London Mayor and Jeremy Corbyn were putting the "crucial relationship at risk", and accused Mr Khan of being a "puffed up pompous popinjay".
A Number 10 spokesman insisted the "strong and deep" partnership between the UK and US "will endure" despite Mr Trump revealing in a trademark tweet that he had cancelled plans to open the new American embassy in London.
Mr Trump said he thought the embassy's move from Grosvenor Square in the prestigious Mayfair district of central London to an "off location" at Nine Elms, south of the Thames, was a "bad deal".
But Mr Khan said the US president's visit would have been met with "mass peaceful protests".
Mr Trump tweeted: "Reason I cancelled my trip to London is that I am not a big fan of the Obama Administration having sold perhaps the best located and finest embassy in London for "peanuts," only to build a new one in an off location for 1.2 billion dollars.
"Bad deal. Wanted me to cut ribbon-NO!"
Reacting to the announcement, Mr Khan, who clashed with the US president after Mr Trump attacked his handling of the London Bridge terror attack, said: "It appears that President Trump got the message from the many Londoners who love and admire America and Americans but find his policies and actions the polar opposite of our city's values of inclusion, diversity and tolerance."
Then Mr Johnson tweeted: "The US is the biggest single investor in the UK - yet Khan & Corbyn seem determined to put this crucial relationship at risk. We will not allow US-UK relations to be endangered by some puffed up pompous popinjay in City Hall."
The Foreign Secretary's post came in the middle of a regular Westminster briefing by a Number 10 spokesman.
Despite being read the tweet, the spokesman was unable to say whether Mr Johnson spoke for the Government, but promised to "look into it".
Asked if Mr Johnson was speaking for the Government, the spokesman replied: "I'm not aware of the circumstances of the tweet, we will come back."