As UK Prime Minister Boris Johnson remains in an intensive care unit at a London hospital with a severe case of coronavirus, messages of support poured in from across the world for the Conservative Party leader.

Johnson was moved to the unit at St Thomas' hospital in central London on Monday, and has been receiving oxygen support.

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In Tuesday's daily afternoon briefing, the de facto deputy prime minister, Dominic Raab, told reporters that Johnson was breathing without assistance and had not required respiratory support.

"He remains in good spirits," said Raab. "And in keeping with usual clinical practice, his progress continues to be monitored closely.

"I'm confident he will pull through. If there is one thing I know about this prime minister, he's a fighter."

It was also announced on Tuesday afternoon that, as of 1600 GMT on Monday, 6,159 people in the UK had died from the coronavirus, according to the health ministry. More than 55,000 people have so far tested positive for the virus.

US President Donald Trump said late on Monday that Americans were praying for Johnson's recovery.

"I've gotten to know him [Johnson]. He's just such an incredible guy," Trump said. "It was just so shocking to see that because you know what that means - intensive care is a big deal with regard to what we're talking about. That's a very big deal. Very scary deal."

Russian President Vladimir Putin on Tuesday wished Johnson a speedy recovery, saying his "optimism and sense of humour" would help him get better.

"I would like to express my sincere support at this difficult moment for you," the Kremlin quoted Putin as saying in a message to Johnson.

"I am sure that your energy, optimism and sense of humour will help to defeat the disease."

French President Emmanuel Macron hoped Johnson would "overcome this ordeal quickly".

"All my support for Boris Johnson, his family and the British people at this difficult time," said Macron.

Canadian Prime Minister Justin Trudeau also said he wished "for a full and speedy recovery" for his British counterpart.

"My thoughts are with you and your family right now. Hope to see you back at Number 10 soon."

Qatar's Emir Sheikh Tamim bin Hamad Al Thani tweeted, in Arabic: "My best wishes for a speedy recovery for my friend, Prime Minister @BorisJohnson. I express my solidarity to you and the Qatari people are with you and the friendly British people at this difficult time."

Afghan President Ashraf Ghani tweeted: "I wish PM @BorisJohnson speedy recovery in the fight against #COVID19. He remains in our prayers and we hope to see him in good health soon. Together with help from our citizens and health workers, we can defeat this common menace."

Benjamin Netanyahu said that the people of Israel were praying for Johnson, who he described as "our friend".

Prime Minister Benjamin Netanyahu and the People of Israel pray for the speedy and full recovery of our friend British Prime Minister @BorisJohnson. 🇮🇱🇬🇧 pic.twitter.com/gNoPE4ulr1 — Benjamin Netanyahu (@netanyahu) April 6, 2020

Irish Prime Minister Leo Varadkar said: "Boris Johnson is in our thoughts ... We wish him a speedy recovery and a rapid return to health."

Michel Barnier, the chief European Union Brexit negotiator, said his thoughts were with Johnson and his family. Carrie Symonds, Johnson's 32-year-old partner who is pregnant, said she had coronavirus symptoms but was feeling better.

Britain has no formal succession plan should the prime minister become incapacitated, but Johnson has asked Raab, the foreign secretary, to stand in for him while he is sick.

Religious leaders also paid tribute to Johnson.

"Saddened to hear that our PM Boris Johnson has been taken into intensive care. My thoughts are with him and his loved ones, wishing him a speedy recovery. We know he will receive the best care and attention from our world class #NHS," said Harun Khan, Secretary General of the Muslim Council of Britain.

"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 Boris Johnson and his loved ones," said Justin Welby, Archbishop of Canterbury.

The UK's Chief Rabbi, Ephraim Mirvis, said: "May Almighty God bless our prime minister as he battles against Covid-19 in hospital. May he be strengthened and granted a swift and complete recovery, together with all those who continue to suffer at the hands of this terrible virus."

Queen Elizabeth II had been kept informed by Downing Street, Buckingham Palace said.

Keir Starmer, the new leader of the Labour Party, the United Kingdom's opposition, called the latest developments "terribly sad news".

"All the country's thoughts are with the prime minister and his family during this incredibly difficult time."

David Cameron, a former British prime minister, said Johnson was "in great hands" and that the country wants him "safe, well and back in 10 Downing Street".

Theresa May, Johnson's predecessor, said her "thoughts and prayers are with Boris Johnson and his family as he continues to receive treatment in hospital".

London Mayor Sadiq Khan, who has clashed politically with Johnson, said he was "praying for the prime minister's swift recovery tonight".

"Guy's and St Thomas' NHS has some of the finest medical staff in the world and he couldn't be in safer hands."

Scotland's First Minister Nicola Sturgeon meanwhile, said she was sending "every good wish" to Johnson.

First leader hospitalised with coronavirus

On March 27, Johnson announced that he had tested positive for COVID-19.

Medics have said the condition of patients with COVID-19 can deteriorate after about 10 days, with some developing pneumonia.

Johnson said recently that he wanted to lose weight. He plays tennis and while mayor of London used to cycle around the capital.

He was first admitted to hospital on Sunday.

The face of the 2016 Brexit campaign, Johnson won a resounding election victory in December before leading the United Kingdom out of the European Union on January 31.

He had faced criticism for insisting on a more modest response to the new coronavirus outbreak than other European leaders, saying on March 3 that he had been shaking hands with coronavirus patients.

He changed tack when scientific projections showed the strategy could have led to a quarter of a million deaths in the United Kingdom.

On Monday, health officials said Britain's death toll stood at 5,373.

Health Minister Matt Hancock also tested positive for the virus last month, as did chief medical adviser Chris Whitty, who also self-isolated. Both appear to have recovered.