The UK Prime Minister, who is 55, was moved into the ward at St Thomas' Hospital after his coronavirus symptoms worsened.
Deputy Gavin St Pier joined leaders around the world in conveying messages wishing him well and a speedy recovery.
US President Donald Trump sent his best wishes to the prime minister on Monday and told a press briefing that: 'All Americans are praying for him, he’s a friend of mine, he’s a great gentleman and a great leader'.
French President Emmanuel Macron tweeted on Monday: 'I send all my support to Boris Johnson, to his family and to the British people at this difficult moment. I wish him a speedy recovery at this testing time.'
Meanwhile, Germany’s government spokesman tweeted a photo of Chancellor Angela Merkel and Boris Johnson together, and expressed her wish for Johnson to have a fast recovery.
The pandemic has eclipsed geopolitical tussles with old enmities laid aside as countries muster all their resources to fight the virus.
The UK was meant to be negotiating a post-Brexit trade deal with the EU but that seems to have been sidelined as governments concentrate on the virus.
The prime minister tested positive for the coronavirus on March 27 and was said to be suffering from 'mild symptoms'.
But Mr Johnson was admitted to hospital in London on Sunday evening for 'tests' due to his persistent symptoms'.
A statement from Downing Street said that 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.'
Speaking on the BBC, the Cabinet Minister Michael Gove said that Mr Johnson had not been put on a ventilator, but had been given oxygen.
The Foreign Secretary Dominic Raab was asked to deputise for the PM and arriving at No 10 on Tuesday, he said the prime minister was 'in very good hands'.
Former Conservative leader Sir Iain Duncan Smith said Mr Johnson's experience underlined the severity of the pandemic: 'if the most powerful man in Britain can come down with this, so can you'.
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