Britain has entered a distressing new phase in its fight against the spread of coronavirus, with the Prime Minister among thousands who have fallen seriously ill.

Boris Johnson is in intensive care with worsening COVID-19 symptoms which means he might not be able to run Britain during the height of the pandemic.

After initially telling people he had a "mild" case, his condition has clearly deteriorated.

He's now in a critical care bed, although we're being told he is still conscious and breathing on his own.

His pregnant fiancee, Carrie Symonds, cannot visit him as she is herself in self-isolation with COVID-19 symptoms.

Boris Johnson's pregnant partner Carrie Symonds is in self-isolation. ( Reuters: Toby Melville )

With Mr Johnson in intensive care, it raises questions over what happens to the governance of 66 million people in one of the biggest crises since World War II.

We've had a look at who is in charge and whether they have the same powers as the Prime Minister.

Britain doesn't really have a line of succession

Despite running against each other for the Tory leadership in 2019, Boris Johnson and Dominic Raab are considered close allies. ( Reuters: Peter Nicholls )

The line of succession for the Prime Minister in the UK is not as clear as it is in other countries, such as the US.

There is actually no official constitutional post of deputy prime minister, so unlike in Australia, there's no obvious stand-in position in a situation like this.

Instead, Downing Street named Mr Raab as "designated survivor" on March 23 when Mr Johnson first fell ill.

Did you just say designated survivor?

Yes, but it's not quite the same as the TV show.

As well as being the Foreign Secretary, Mr Raab is also the First Secretary of State, which makes him the obvious pick to be designated survivor and stand-in PM if the situation becomes graver.

Mr Raab was first elected to Parliament in 2010 after working as an adviser to the Foreign Office. ( Reuters: Peter Nicholls )

However, there can be some dispute and competition over who can hold the position in the UK.

According to local media reports, Cabinet Office Minister Michael Gove was pushing to be the PM's stand-in, so Downing Street was forced to make a choice.

That was a huge concern for Peter Bone, a Conservative backbencher who has been calling for a law to clarify the line of succession in the UK since 2013.

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"Nobody seems to be able to tell me what happens if the Prime Minister is incapacitated," he told the Mirror newspaper.

"In a national emergency, you don't want to be scrabbling around worrying about who's in charge. And you don't want the Foreign Secretary and the Cabinet Office Minister arguing about who's in charge."

The situation is now so serious that the Queen is being regularly informed of the Prime Minister's condition.

The Queen's rallying speech to the nation on April 6 was dimmed somewhat by the news an hour later that Mr Johnson had been taken to hospital.

What kind of powers does Dominic Raab have?

There are various things that only the UK Prime Minister is authorised to do.

Some of these responsibilities could be taken up by Mr Raab without any issue, including making recommendations to the Queen on appointments to the senior judiciary, and to the Church of England.

Those kinds of tasks for the moment are unlikely to be pressing as most of the nation remains on lockdown.

But there could be some times where the authority is not as clear cut.

For example, according to the BBC it is harder to imagine him conducting a cabinet reshuffle — even though, formally, it seems he ought to have that power.

And things seem to get even less likely as we get to the more significant prerogative powers.

Mr Raab said a team of ministers and Whitehall officials were working "full throttle" to carry out the instructions of Mr Johnson from his hospital bed at St Thomas' Hospital in central London.

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But when asked whether he had taken over Mr Johnson's "security responsibilities," Mr Raab declined to comment.

"We are getting on with all of the various strands of work to make sure at home and abroad we can defeat the virus and pull the country through coronavirus and the challenges that undoubtedly we're facing at the moment," he added.

So, who is Dominic Raab?

Mr Raab is seen to be skilled and combative but has courted some controversy in the past. ( ABC News: Sean Davey )

Mr Raab entered the Conservative Party race to become leader last year but finished a dismal sixth in the contest before quickly backing Mr Johnson.

Months later he finds himself overseeing the response to Britain's biggest crisis since World War II.

Like the Prime Minister, he's a vocal Brexiteer.

Both men supported the "Vote Leave," pro-Brexit campaign, before the referendum on the European Union in 2016.

Mr Raab was first elected to Parliament in 2010 after working as an international lawyer and an adviser to the Foreign Office.

He was a junior minister in former prime minister Theresa May's government before he was promoted to Brexit secretary in 2018.

But he quickly became a thorn in her side and resigned several months later in protest over a deal she struck with the EU on Brexit.

Mr Raab is seen to be skilled and combative but has courted some controversy in the past.

During the leadership campaign, previous comments he'd made on feminism were raised again by journalists.

"From the cradle to the grave, men are getting a raw deal," he said in 2011.

"Men work longer hours, die earlier, but retire later than women."

He defended those comments in 2019, saying there shouldn't be "double standards" when discussing equality between the sexes.

So, what happens next?

Boris Johnson was transferred to intensive care on Monday. ( Reuters: Hannah Mckay )

Most UK parliamentary experts believe that if Mr Johnson were totally incapacitated, Mr Raab would be his stand-in until the Tories elect a new leader and take Britain to an election.

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In normal times, grassroots members of the Conservative Party would vote for a new leader, as well as MPs.

For now, though, Mr Raab insists that he is only carrying out instructions he receives from Mr Johnson in hospital.