Scotland's first minister has said her thoughts are with Boris Johnson and his family after the prime minister was taken to intensive care.
Mr Johnson was admitted to hospital on Sunday because he was still suffering coronavirus symptoms 10 days after testing positive.
His condition is said to have worsened, and he was moved to intensive care on Monday evening.
Nicola Sturgeon tweeted that she was "sending every good wish" to the PM.
A spokesman for the prime minister said he was taken to intensive care on the advice of his medical team, and had asked Foreign Secretary Dominic Raab to deputise "where necessary".
A No 10 statement read: "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 [Monday] 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."
It continued: "The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication."
Mr Johnson, 55, is understood to have been given oxygen on Monday afternoon before being taken to intensive care as a precaution in case he needed to be given ventilation. He is also understood to be conscious.
He was initially taken to hospital for routine tests after testing positive for coronavirus 10 days ago. His symptoms included a high temperature and a cough.
Earlier on Monday, he tweeted that he was in "good spirits".
My thoughts are with the PM and his family - sending him every good wish https://t.co/tjpadJq6bq— Nicola Sturgeon (@NicolaSturgeon) April 6, 2020
Scottish Conservative leader Jackson Carlaw said it was "desperately worrying news" and sent his "heartfelt good wishes" to Mr Johnson and his fiancée Carrie Symonds, who is pregnant.
Mr Carlaw added: "Our thanks to all in the NHS tending to the PM and so many others across the country tonight."
Ms Symonds has also been experiencing symptoms of coronavirus, but said on Saturday that she was feeling "stronger" and was "on the mend".
Scottish Labour leader Richard Leonard also tweeted: "I wish Mr Johnson a speedy recovery."
Intensive care is where doctors look after the sickest patients - his admission to ICU is the clearest indication of how ill the prime minister is.
We do not know the full details of Mr Johnson's condition, but he is conscious and not being ventilated.
Not every patient in intensive care is ventilated, but around two-thirds are within 24 hours of admission with Covid-19.
This is a disease that attacks the lungs and can cause pneumonia and difficulty breathing.
This leaves the body struggling to get enough oxygen into the blood and to the body's vital organs.
There is no proven drug treatment for Covid-19, although there are many experimental candidates.
But the cornerstone of the prime minister's care will depend on getting enough oxygen into his body and supporting his other organs while his immune system fights the virus.