Donald Trump has wished Boris Johnson a "speedy recovery" after the prime minister tested positive for coronavirus.
Mr Johnson, 55, tweeted a video message on Friday morning confirming he has COVID-19, writing: "Over the last 24 hours I have developed mild symptoms and tested positive for coronavirus."
He added: "I am now self-isolating, but I will continue to lead the government's response via video-conference as we fight this virus."
Downing Street announced that Mr Johnson had since spoken to the US president on the phone, with the two leaders committed to working together to "defeat the coronavirus pandemic".
Image: The PM visited a lab to see a model of the virus in early March
"Prime Minister Boris Johnson spoke with US President Donald Trump today," said a spokesman.
"The president wished the prime minister a speedy recovery from coronavirus.
"They agreed to work together closely, along with the G7, the G20, and other international partners, to defeat the coronavirus pandemic."
Mr Trump has previously tested negative for the disease, which has infected more people in the US than in any other country worldwide - including China, where coronavirus first emerged back in December.
While Mr Trump and his top White House officials have so far not tested positive for the coronavirus, the disease has infected several figures at leading the fight against the disease in the UK.
Joining Mr Johnson in announcing his diagnosis on Friday morning was Health Secretary Matt Hancock.
He said on Twitter that he had caught the virus and was self-isolating at home with "mild symptoms".
Downing Street said Mr Johnson had also experienced "mild symptoms" and was tested on Thursday afternoon on the advice of his chief medical officer Professor Chris Whitty.
The professor has since also gone into self-isolation after experiencing symptoms.
The PM's positive result was confirmed at around midnight, Downing Street said.
Image: Matt Hancock was sat next to Boris Johnson on Wednesday
Number 10 also said meals would be left outside Mr Johnson's door while he self-isolates.
And they added because the building was considered a workplace, staff did not need to self-isolate unless they have symptoms - such as a new persistent cough or raised temperature.
Image: Mr Johnson was tested on the advice of his chief medical officer Chris Whitty (left)
Image: Mr Johnson's partner Carrie Symonds is pregnant
Mr Johnson, whose partner Carrie Symonds is pregnant, was last seen outside Number 10 on Thursday night, joining Chancellor Rishi Sunak as part of a nationwide applause for NHS workers as they battle the pandemic.
A Treasury source said Mr Sunak was not self-isolating because he did not have any symptoms, but other people in the prime minister's office are.
Mr Johnson also appeared at PMQs as normal on Wednesday.
He sat in a gap between his Home Secretary Priti Patel and Chief Whip Mark Spencer.
And all of this week he has been chairing the daily "war cabinet" on Britain's COVID-19 emergency response, involving some face-to-face meetings with those at the heart of managing the health, economic and communication response to the crisis.
But Mr Johnson has appeared over videolink for two important calls - with cabinet ministers and other leaders of the G20 countries.
Image: The cabinet met over conference call. Pic: Andrew Parsons/Number 10
He also did not travel to Buckingham Palace for his weekly private meeting with the Queen on Wednesday - instead calling her on the phone. The pair last met on 11 March.
Mr Johnson's senior adviser Dominic Cummings was seen running out of Downing Street after the news broke.
PM's chief adviser seen running from No 10
Image: The Queen held her weekly phone call with Mr Johnson on the phone
Mr Hancock confirmed in a video message on Twitter on Friday he had COVID-19 after showing "very mild" symptoms and is now self-isolating while working from home.
It is "vital we follow the advice to protect our NHS and save lives", he warned.
Back on 3 March, the prime minister revealed in a news conference he shook hands with COVID-19 patients.
"I was at a hospital the other night where there were actually a few coronavirus patients and I shook hands with everybody, you'll be pleased to know," he said.
'I shook hands with everybody' at coronavirus hospital
Sam Coates, Sky News' deputy political editor, said: "This is an enormous challenge for the centre of government today.
"There will be a lot of concern that a large number of people in Number 10 might have been exposed and might not have to be tested."
Mr Johnson's spokesman said a few days ago that Foreign Secretary Dominic Raab would stand in if he becomes incapacitated.
But they confirmed on Friday Mr Raab is not taking over any functions from the prime minister yet.
Image: Dominic Raab will take over if the PM is incapacitated
Jeremy Corbyn wished Mr Johnson a "speedy recovery", tweeting: "I hope his family are safe and healthy.
"Coronavirus can and does affect anyone. Everyone be safe. Our own health depends on everybody else."
Michel Barnier, the EU's chief Brexit negotiator, also wished Mr Johnson well and tweeted: "My thoughts are with him and, indeed, with all families in the UK and across Europe and the world that are affected by coronavirus. I am convinced Europe can overcome these difficulties together."
Fears grew about how far COVID-19 spread at the heart of government after health minister Nadine Dorries tested positive for the virus earlier in March.
She has since recovered and was clapped by MPs when she returned to the Commons on Wednesday to hear the health secretary make a statement.
DGJaguar on March 27th, 2020 at 11:24 UTC »
His latest tweet. It's concerning because it now seems likely other cabinet members would get it too.
stay-puft-mallow-man on March 27th, 2020 at 11:23 UTC »
Is this the biggest name to test positive?
rabidstoat on March 27th, 2020 at 11:21 UTC »
Biggest world leader so far to get it. He'll need to work remotely for the most part over the next couple of weeks.