British Prime Minister Boris Johnson remains in intensive care after his coronavirus symptoms "worsened".
He had been moved to intensive care on Monday after his condition deteriorated. However, on Tuesday Downing Street said he did not have pneumonia and was in a "stable" condition and "remains in good spirits".
Foreign Secretary Dominic Raab said: “He is not just our prime minister, he is our boss, our colleague and our friend. “I’m confident he’ll pull through because if there’s one thing I know about this Prime Minister, he’s a fighter and he’ll be back at the helm leading us through this crisis in short order.”
Mr Johnson was admitted to London's St Thomas's Hospital on Sunday night.
Initially government officials said he was taken to hospital as a "precautionary measure", and that he remained in charge of the government.
He had been suffering from persistent symptoms including a high temperature and a cough for days.
At 7pm on Monday, Mr Johnson was moved to the intensive care unit at St Thomas's Hospital after his health deteriorated.
He remained conscious but was understood to have been moved as a precaution in case he required ventilation to aid his recovery.
Presently he is receiving oxygen but does not require the use of a ventilator.
On Tuesday, Downing Street said: "His condition worsened yesterday afternoon. A decision was taken that he needed to be moved to an intensive care unit at around 7pm.
"We informed you all as soon as was practically possible. We have a commitment to be as transparent as we can be throughout this process."
Earlier, a spokesperson had said: "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."
Mr Raab has taken over the day-to-day running of Britain's response to the pandemic and is chairing emergency meetings on stopping its spread.
On Tuesday, Downing Street revealed Mr Raab has the authority to authorise military action in the absence of the prime minister but he would not undertake Mr Johnson's weekly audience with the Queen, which has now been suspended until he has recovered.
In the event of Mr Raab becoming ill, the next person to take control of the country would be Chancellor Rishi Sunak.
Mr Raab, who has not spoken to Mr Johnson, said on Monday that the government would continue to focus on implementing Mr Johnson's social restrictions to contain the spread of the coronavirus.
The spokesperson has refused to say whether or not the UK lockdown measures will be extended when the initial three-week period ends on Easter Monday.
On Tuesday, 758 people died in England overnight from the virus.
Mr Raab has said there is an "incredibly strong spirit behind the prime minister".
"Since Sunday evening, the Prime Minister has been under the care of doctors at St Thomas’s Hospital, in London, after being admitted with persistent symptoms of coronavirus," Downing Street said.
"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."
Former British prime minister David Cameron said on Twitter he was thinking of Mr Johnson and his family.
"Thinking of Boris 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," he said.
The leader of the opposition Labour party, Keir Starmer, said it was "terribly sad news" and that all of the UK's thoughts were with Mr Johnson and his family during this "incredibly difficult time".
Earlier on Monday, Mr Johnson tweeted that he "was in good spirits".
Dr Anwar Gargash, the UAE's Minister of State for Foreign Affairs, said his thoughts and prayers were with Mr Johnson.
"Wishing him a speedy and full recovery. He is a resilient fighter and will pull through," Dr Gargash said on Twitter.
US President Donald Trump sent his "best wishes to a very good friend of mine", while leaders in Europe, India, Canada and Australia urged his swift recovery.
Russian President Vladimir Putin said Johnson's "energy, optimism and sense of humour" would help him get better.
French President Emmanuel Macron tweeted: "All my support for Boris Johnson, his family and the British people at this difficult time. I wish him to overcome this ordeal quickly."