This evening Boris Johnson was transferred to an intensive care unit as he battles coronavirus he was first diagnosed with 10 days ago.
Downing Street confirmed in a statement that Boris Johnson's condition had worsened following his admission to St Thomas' Hospital on Sunday with "persistent" symptoms.
He was said to have been taken to hospital 'as a precaution' .
Hours before his transfer to ICU the 55-year-old PM, who tested positive for Covid-19 on March 26, wrote that he was in "good spirits" and praised the care he was receiving.
But at around 7pm he was moved to the ICU after his breathing deteriorated.
He had required oxygen this afternoon and the decision was made to have him closer to the ventilators that may be required if he needs help breathing.
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Allies and opponents alike issued messages of support, with new Labour Party leader Keir Starmer posting on Twitter: "Terribly sad news. All the country’s thoughts are with the Prime Minister and his family during this incredibly difficult time."
The Queen is being kept up-to-date on Mr Johnson's condition, Buckingham Palace confirmed.
In a statement Downing Street said that Foreign Secretary and First Minister Dominic Raab has been instructed to deputise for Mr Johnson as needed.
Mr Johnson's pregnant fiancée, Carrie Symonds, 32, has also been suffering with coronavirus symptoms and spent the past week in bed.
What do we know about the PM's condition?
Cabinet members were told that Mr Johnson was conscious and not on a ventilator.
He did however require oxygen as his breathing worsened.
Insiders said the virus - which has killed more than 5,000 in the UK - hit the PM "like a sledgehammer" but there had been concerns over the weekend that he was not taking the sufficient rest to recover from Covid-19.
Earlier Mr Raab said Mr Johnson continued to lead the government from his hospital bed.
What has Downing Street said?
A statement from Number 10 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."
What will happen to Mr Johnson in intensive care?
There is no definite answer to this, and officials have claimed that the move was made as a precaution.
Professor Derek Hill, professor of medical imaging at University College London (UCL), said it was "very likely" Mr Johnson would be put on a mechanical ventilator.
He said: "It seems clear that the Prime Minister went to hospital because he had difficulty breathing.
"It seems he was initially put on oxygen, and was conscious.
"But as often happens with Covid-19, his condition has now deteriorated so he has been admitted to intensive care where he is very likely to have been put on a mechanical ventilator to breath for him."
Who will run the government in the PM's absence?
Foreign Secretary and First Minister Dominic Raab has been instructed to deputise for Mr Johnson - although he will not be named acting Prime Minister.
Mr Raab - who last spoke to the PM on Saturday - told the BBC: "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."
What has Mr Johnson himself said since his diagnosis?
On Monday the Prime Minister wrote on Twitter : "I'm in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe."
In a video posted on Twitter on Friday, Mr Johnson said he would remain in isolation because he still had a temperature.
He said: "Although I’m feeling better and I’ve done my seven days of isolation, alas I still have one of the symptoms, a minor symptom, I have a temperature.
"So in accordance with government advice I must continue my self-isolation until that symptom itself goes.”
He appeared on the steps of 10 Downing Street on Thursday evening to applaud NHS workers for their bravery responding to the coronavirus crisis.
What has the reaction been to the news that the PM is in intensive care?
As news of his condition worsening broke, Labour leader Keir Starmer wrote on Twitter : "Terribly sad news. All the country’s thoughts are with the Prime Minister and his family during this incredibly difficult time."
And Jeremy Corbyn, who stood down as leader of the opposition at the weekend, posted: "My thoughts are with Boris Johnson and his family tonight. Thanks to the NHS staff for their hard work and dedication."
Former Prime Minister David Cameron wrote: "Thinking of Boris Johnson and his family tonight. Get well soon. You are in great hands and we all want you safe, well and back in 10 Downing Street."
And Theresa May said: "My thoughts and prayers are with Boris Johnson and his family as he continues to receive treatment in hospital."
Chancellor Rishi Sunak wrote: "My thoughts tonight are with Boris Johnson and (his fiancee) Carrie Symonds. I know he'll be getting the best care possible and will come out of this even stronger."
And French President Emmanuel Macron wrote: "All my support for Boris Johnson, his family and the British people at this difficult time. I wish him a speedy recovery from this ordeal."
How is the Prime Minister's fiancee?
Boris Johnson's fiancée Carrie Symonds is facing the agony of being unable to be by the Prime Minister's side while he is in intensive care.
Ms Symonds, who is around six months pregnant, is reported to be self-isolating with the couple's dog just miles from his hospital ward in London after suffering with coronavirus symptoms herself.
The 32-year-old has been apart from Mr Johnson for around two weeks following his coronavirus Covid-19 diagnosis.
She tweeted: "I’ve spent the past week in bed with the main symptoms of coronavirus.
"I haven’t needed to be tested and, after seven days of rest, I feel stronger and I’m on the mend."