Boris Johnson's GP ordered him to go to hospital after a video check-up showed how bad his coronavirus symptoms were, according to reports.
The PM - who was isolating in the flat at 11 Downing Street - was deteriorating by the hour on Sunday, coughing and struggling to breathe, it has been said.
He had initially attempted to put a brave face on his fight against the deadly bug, continuing to work long hours during his 10 days in quarantine.
But he decided to consult his doctor after feeling worse following an afternoon nap, a source told the Daily Mail.
On waking up, the newspaper reports he felt no better than he had when he'd woken that morning and consulted his doctor via video link.
The practitioner recognised Mr Johnson was far worse than when they last spoke earlier in the week - with the PM having ignored his orders to get plenty of rest.
Mr Johnson had admitted he was finding breathing difficult and having suffered with pneumonia-like symptoms previously, the doctor was keen to get him to hospital.
However, there is currently no suggestion the Prime Minister has pneumonia now.
A different source told the Mail that you can't "underestimate the macho nature of the Westminster political establishment".
They said Mr Johnson "will not have wanted to look weak" but after speaking to his GP he eventually relented.
After apparent minor arguments with his officials, at 8pm he was taken by car to St Thomas' Hospital in central London.
Last night, Downing Street then confirmed the PM had been taken into intensive care with his condition having worsened.
Mr Johnson was then moved to the ICU at around 7pm last night after experiencing breathing difficulties which required him to be given oxygen.
Foreign Secretary Dominic Raab - who earlier said the PM was in "good spirits" - has been instructed to deputise for Mr Johnson.
Buckingham Palace said the Queen is being kept up-to-date on Mr Johnson's condition.
Just hours before he was moved, Mr Johnson tweeted: "I'm in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe."
