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U.K. Prime Minister Boris Johnson’s move to intensive care on Monday is raising fears about a potential paralysis at the heart of British government in the middle of a national crisis -- particularly as the British system has no clearly defined line of succession in place if a prime minister should become incapacitated.

Johnson was moved to intensive care on Monday after being admitted to a hospital on Sunday because of worsening coronavirus symptoms. Downing Street said Tuesday that Johnson is “in good spirits,” is conscious and is not on a ventilator.

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Foreign Secretary Dominic Raab has been deputized "where necessary" in Johnson’s absence and will be conducting daily cabinet meetings as the government’s response to the crisis continues.

As foreign secretary, Raab is formally the First Secretary of State, which gives him a certain superiority over other cabinet members, and therefore a natural pick for a successor in case of an emergency.

The Daily Telegraph reported that Downing Street foresaw a possible constitutional dilemma and started drawing up a "designated successor" plan with Raab picked as first successor.

WHO IS DOMINIC RAAB?

According to the Telegraph, Chancellor of the Exchequer Rishi Sunak would be next in line if Raab was taken ill. That is not a far-fetched proposition given the rapid spread of the virus. Health Secretary Matt Hancock tested positive for the virus last month and Cabinet Minister Michael Gove is currently self-isolating after a family member tested positive for it.

Should Johnson be incapacitated for a longer time or permanently, it would likely eventually see a new leadership election in the governing Conservative Party, and whoever won that would take over permanently as prime minister.

This is what normally happens when a prime minister resigns. Johnson took over 10 Downing Street from Theresa May last year after a six-week leadership election following her resignation. May had also taken the keys to Number 10 after winning a leadership election following the resignation of then-Prime Minister David Cameron in 2016.

But it is not clear to what extent, if at all, a prime minister’s death would change that dynamic, especially during an unprecedented crisis like the coronavirus -- which has locked down much of British life.

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There is little past precedent to go on. The last party leader to die in office was Labour Party leader John Smith, who died suddenly of a heart attack in 1994. But he was in opposition, not prime minister, and that allowed a full leadership race to go ahead.

The last prime minister to die in office was Lord Palmerston in 1865. He was succeeded by then-Foreign Secretary Lord John Russell.

More recently, Prime Minister Winston Churchill suffered a stroke in the 1950s, while Prime Minister Tony Blair was briefly hospitalized for heart surgery in 2004 -- but neither handed over their powers, even temporarily.

CORONAVIRUS: WHO IS BRITISH FOREIGN SECRETARY DOMINIC RAAB?

With Raab deputized, it is not entirely clear to what extent he holds the powers of the prime minister. Unlike in the U.S., Britain has an unwritten constitution, and therefore while there is guidance in documents such as the Cabinet Manual -- it is not always explicitly set out what must happen in extreme circumstances. Additionally, the British head of state is Queen Elizabeth II and not the prime minister.

That uncertainty means therefore that Raab’s role could have some limits. With Johnson conscious, it is not clear how much of his job has been deputized to Raab. Raab can make recommendations to Her Majesty on appointments to the senior judiciary and elsewhere according to the BBC, but may fall short of being able to reshuffle the Cabinet, although he may have the power to do so.

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Less clear would be what to do about the so-called letters of last resort written for nuclear submarines in the case of a serious attack. Those and other military-related decisions would likely be taken in consultation with other Cabinet ministers.

But in the meantime, Raab is likely to postpone making major decisions as long as possible, such as the lifting of the national lockdown, until a hopeful return to power soon by Johnson.

"Government will always continue. The people are there, the support's there,” Speaker of the House of Commons Lindsay Hoyle told the BBC on Tuesday. “Whatever happens, no matter how bad it is, the country continues, government continues.”