That the leader of one of the most powerful nations and largest economies in the world is now in intensive care with worsening symptoms of coronavirus is extraordinary.

Downing Street says Boris Johnson's condition worsened over the course of Monday afternoon, and his medical team advised that he be moved to ICU at St Thomas' Hospital in central London.

That decision would not have been taken lightly.

Downing Street had spent the past 10 days assuring the public that the Prime Minister was fit to govern and had "mild" symptoms of COVID-19.

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The word "mild" was dropped in recent days.

Just hours before he was admitted to intensive care, Downing Street was insisting Mr Johnson was still well enough to govern.

But was he really?

Sure enough, something was not right

The scepticism began to grow after Mr Johnson was admitted to hospital on Sunday, an announcement that was made just after the Queen had completed her special address to the nation.

By Monday, the questions came thick and fast during the daily coronavirus update that was led by Foreign Secretary Dominic Raab.

A statement from Downing Street said Dominic Raab had been asked to deputise "where necessary". ( ABC News: Sean Davey )

He was questioned repeatedly about whether the PM could continue to capably lead the country during a time of crisis while in hospital.

Mr Raab refused to answer questions about tests and treatment, and on one occasion pointed to national security as a reason for no comment.

He repeatedly said the British leader was in "good spirits".

Then Mr Raab revealed he had not spoken to Mr Johnson since Saturday, one day before the Prime Minister was admitted to hospital, despite standing in for him on Monday's daily government coronavirus briefing.

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That raised the suspicions of some of the most seasoned political journalists in the UK, including the BBC's Andrew Neil, who tweeted: "Why has Mr Raab not spoken to PM since Saturday? Something not right here."

Sure enough, something was not right.

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Was Johnson taken to hospital quickly enough?

Hours later came the bombshell that Boris Johnson had been transferred to intensive care.

A statement from Downing Street said the PM had asked Mr Raab to deputise for him "where necessary".

That means Mr Raab is now the interim head of government, because Boris Johnson is likely no longer fit enough to lead.

He is not the deputy prime minister as it would be in Australia, but rather the First Secretary of State.

Regardless, he can make decisions on Mr Johnson's behalf, including on security and the coronavirus response.

Over the past 11 days, it's clear Downing Street went to extraordinary lengths to assure the people their leader was fine.

Even on Monday night, the message was that the move to ICU was precautionary — Mr Johnson was not on a ventilator, and was conscious when he was moved there.

But the questions will come as to whether he was moved to hospital quickly enough, and whether the public was told the truth of his condition.

For now, his battle is a very personal one

Medical experts have regularly pointed to a deterioration among some patients at the 10-day mark of the virus. Others, like Health Secretary Matt Hancock, seem to sail through with little drama.

Mr Hancock was diagnosed at the same time as Mr Johnson.

He is 14 years younger than the British leader, and returned to work and started fronting press conferences days ago.

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Since then, Mr Hancock has been front of the campaign to urge people to stay home as the weather in the UK improves after a long winter.

Mr Johnson's turn for the worse will likely focus the minds of those who have perhaps been taking the threat too lightly.

For now, his battle is a very personal one.