Boris Johnson has been moved to intensive care after his coronavirus symptoms worsened.
The Prime Minister was admitted to St Thomas’ Hospital in London on Sunday after his Covid-19 symptoms persisted for ten days.
Johnson is understood to be conscious and was moved to intensive care at about 7pm on Monday as a precaution should he require a ventilator.
Meanwhile, the Prime Minister’s pregnant fiancee, Carrie Symonds, earlier said she was “on the mend” after spending a week in bed with coronavirus symptoms.
The 32-year-old, who is expecting the couple’s baby in early summer, has been self-isolating in Camberwell, south London, with the couple’s dog Dilyn.
Foreign secretary Dominic Raab will now “deputise” for the Prime Minister “where necessary”.
A Number 10 spokesman said: “Since Sunday evening, the Prime Minister has been under the care of doctors at St Thomas’ Hospital, in London, after being admitted with persistent symptoms of coronavirus.
“Over the course of this afternoon, the condition of the Prime Minister has worsened and, on the advice of his medical team, he has been moved to the intensive care unit at the hospital.
“The PM has asked foreign secretary Dominic Raab, who is the First Secretary of State, to deputise for him where necessary.
“The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication.”
First Minister Nicola Sturgeon tweeted that her “thoughts are with the PM and his family – sending him every good wish”.
The SNP added: “Our and everyone’s thoughts at the SNP are with the Prime Minister and his family right now.
“We hope and wish for a speedy recovery.”
Scottish Tory leader Jackson Carlaw said it was “desperately worrying news” and sent “heartfelt good wishes” to the PM and his partner.
The MSP added: “And our thanks to all in the NHS tending to the PM and so many others across the country tonight.”
At a White House press conference, US President Donald Trump said: “Before I begin, I want to express our nation’s well wishes to Prime Minister Boris Johnson as he wages his own personal fight with the virus.
“All Americans are praying for him. He’s a friend of mine. He’s a great gentleman and a great leader, and as you know he was brought to the hospital today, but I’m hopeful and sure that he’s going to be fine.
“He’s a strong man, a strong person.”
Earlier in the afternoon, Johnson said he was in “good spirits” after spending the night in hospital.
He tweeted: “Last night, on the advice of my doctor, I went into hospital for some routine tests as I’m still experiencing coronavirus symptoms. I’m in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe.
“I’d like to say thank you to all the brilliant NHS staff taking care of me and others in this difficult time. You are the best of Britain.”
At a press conference after the decision was made, Raab said the Prime Minister was in “safe hands” at St Thomas’ Hospital.
Raab said there was an “incredibly strong team spirit behind” Johnson and that ministers were determined to carry out his “direction”.
“The Government’s business will continue,” he said.
“The Prime Minister is in safe hands with that brilliant team at St Thomas’ Hospital, and the focus of the Government will continue to be on making sure that the Prime Minister’s direction, all the plans for making sure that we can defeat coronavirus and can pull the country through this challenge, will be taken forward.”
He added: “There’s an incredibly strong team spirit behind the Prime Minister, and making sure that we get all of the plans the Prime Minister’s instructed us to deliver, to get them implemented as soon as possible.
“And that’s the way it will bring the whole country through the coronavirus challenge that we face right now.”
The Queen has been kept informed by Downing Street about the Prime Minister’s condition, Buckingham Palace said.
A Palace spokesman said: “Her Majesty has been kept informed by No 10.”
Johnson is the 14th prime minister of the Queen’s reign.