BORIS Johnson is now engaged in a serious personal battle against coronavirus after he was moved to hospital intensive care following a worsening of his condition.

The 55-year-old Prime Minister was understood to have still been conscious when he was admitted to the intensive care unit at St Thomas’s Hospital in Westminster around 7pm, a move made as a precaution should he need a ventilator to help with his breathing.

The deterioration in his condition appeared to have happened rapidly after earlier in the day Mr Johnson tweeted bullishly that he was in “good spirits” and praised the NHS staff caring for him, describing them as the “best of Britain”.

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But late on Monday afternoon his condition worsened and it is believed he was given oxygen to help with his breathing before being moved to the intensive care unit at St Thomas’s.

The PM had been admitted just 24 hours earlier on Sunday evening after he was unable to shake off a persistent high temperature after 10 days in self-isolation.

His worsening condition will cause deep concern across the whole country and not least among his family, friends and colleagues at Westminster. Any return to frontline Government work now looks highly unlikely any time soon.

Last night, two uniformed police officers were stationed at the entrance to St Thomas's Hospital. Throughout the day officers and police vehicles could be seen entering and exiting the central London hospital.

The PM made clear that Dominic Raab, the Foreign Secretary, who earlier in the day had chaired the so-called Covid-19 “war cabinet,” should deputise for him where necessary. Mr Raab is also the First Secretary of State and so, technically, second in the chain of command.

In a statement recorded after Mr Johnson's admission, he said: "The Government's business will continue.

"The Prime Minister is in safe hands with that brilliant team at St Thomas's 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."

Buckingham Palace said the Queen was being kept informed by Downing Street about the condition of Mr Johnson, the 14th prime minister of her reign.

At just after 8pm No 10 issued a statement, revealing the latest serious turn of events.

It said: “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.

“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,” it added.

Messages of support quickly poured in from politicians at home and abroad.

Chancellor Rishi Sunak tweeted: "My thoughts tonight are with @BorisJohnson and @carriesymonds. I know he'll be getting the best care possible and will come out of this even stronger."

Alister Jack, the Scottish Secretary, told The Herald: “I wish him a speedy recovery. My thoughts are with him and I look forward to seeing him back at his desk.”

David Cameron tweeted: "Thinking of @BorisJohnson and his family tonight. Get well soon. You are in great hands and we all want you safe, well and back in @10DowningStreet."

Theresa May, the PM’s predecessor, also took to social media to say: “My thoughts and prayers are with @BorisJohnson and his family as he continues to receive treatment in hospital. This horrific virus does not discriminate. Anyone can get it. Anyone can spread it. Please #StayHomeSaveLives."

Ruth Davidson, the former Scottish Conservative leader, said: “The whole country will be wishing the PM a speedy recovery to full health and thanking the expert NHS medical staff, who are looking after him and so many others at this time.

“He is getting the best care possible and we hope to see him back in No 10, full of his usual energy and ebullience, at the earliest possible opportunity.”

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The MSP added: “Our thoughts too are with Carrie, herself recovering from symptoms and expecting their first child.”

Nicola Sturgeon, the First Minister, tweeted: "My thoughts are with the PM and his family; sending him every good wish."

Sir Keir Starmer, the New Labour leader, posted: "Terribly sad news. All the country's thoughts are with the Prime Minister and his family during this incredibly difficult time."

Mayor of London Sadiq Khan tweeted: "Praying for the Prime Minister's swift recovery tonight. @GSTTnhs has some of the finest medical staff in the world, and he couldn't be in safer hands."

Nigel Farage, the Brexit Party leader, also took to Twitter, saying: "I hope and pray for @BorisJohnson to come through this ordeal quickly."

Sir Lindsay Hoyle, the Commons Speaker, said: "This is terrible news. I know the thoughts and prayers of everyone across the House are with the Prime Minister and his family right now. We all wish him a speedy recovery."

Justin Welby, the Archbishop of Canterbury, tweeted: "The news that our Prime Minister has been moved to intensive care deepens our compassion for all who are seriously ill and for those caring for them. I invite all people of faith to join me in praying for @BorisJohnson and his loved ones."

From abroad, a string of world leaders also sent their messages of sympathy and support via social media.

Donald Trump said Americans were praying for Mr Johnson's recovery.

The US President told reporters at the White House that the British leader had "been a really good friend".

He declared: "He's been really something very special, strong, resolute, doesn't quit, doesn't give up."

Mr Trump added: "When you get brought into intensive care, that gets very, very serious with this particular disease."

Emmanuel Macron, the French President, said: "All my support to Boris Johnson, his family and the British people at this difficult time. I wish him well."

Justin Trudeau, the Canadian premier, tweeted: "Sending my best wishes to Prime Minister @BorisJohnson for a full and speedy recovery. My thoughts are with you and your family right now. Hope to see you back at Number 10 soon."

Pedro Sanchez, the Prime Minister of Spain, which has also suffered a high number of cases of coronavirus, posted: "My solidarity and wishes for a speedy recovery for the Prime Minister @BorisJohnson. These are difficult days for our countries, but from strength and unity, we will be able to win this battle. A hug to all the British people."

Guiseppe Conte, the Italian PM, tweeted: "My thoughts go to @BorisJohnson and all the British people. I wish you a fast recovery. You have the whole Italian Government's support and my personal one. We are all confident that your Country will get through this difficult time."