London (CNN) British Prime Minister Boris Johnson is receiving "standard oxygen treatment" and is breathing without assistance, his spokesperson said Tuesday, a day after he was transferred to intensive care with coronavirus .

Johnson's condition is stable and he "remains in good spirits," his spokesperson said. He has not required invasive or non-invasive ventilation and does not have pneumonia.

But the seriousness of the Prime Minister's condition at a time of national emergency, combined with the lack of a formal succession procedure for heads of government in the UK, has raised questions about who is leading the country.

Johnson has nominated his Foreign Secretary, Dominic Raab, to deputize for him "as necessary" but there is no recent precedent for a sitting UK prime minister becoming incapacitated for a lengthy period.

In its daily update to reporters, Downing Street moved to head off concerns, stressing that Raab was running the country from the Foreign and Commonwealth Office with the support of government officials.

Raab would chair meetings of the National Security Council if any were needed while he was deputizing for the Prime Minister, a spokesperson said. Raab, 46, and the UK's Cabinet have the authority and ability to respond in the PM's absence on military action, the spokesperson added.

British police officers stand on duty outside St Thomas' Hospital in central London, where Britain's Prime Minister Boris Johnson is in intensive care on April 7, 2020.

Decisions relating to Covid-19 would be taken in the usual way through the daily morning meeting, chaired by Raab, and other ministerial groupings. But Raab would not have the power to hire and fire ministers or officials, the spokesperson said.

The UK lockdown will remain in place with the government "focused on stopping the spread of the infection." The spokesperson said that if there were evidence that more needed to be done to slow the rate of transmission, "we would be prepared to do that."

Who is running the UK?

Few formal powers are invested specifically in the UK prime minister and key decisions are taken collectively by the Cabinet or its sub-committees. Many statutory powers are held by individual secretaries of state. But in recent decades, holders of the UK's top political office have adopted a more presidential style, and the sweeping nature of the ruling Conservative Party's most recent election victory was attributed to Johnson's personal appeal with voters.

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"The Prime Minister has a team around him who ensure the work of government goes on," a senior Cabinet minister, Michael Gove, told the BBC earlier. He sidestepped a question about who would take decisions in the event of a nuclear attack, saying he would not discuss national security issues.

Conservative MP Tobias Ellwood, who chairs the House of Commons defense select committee, tweeted good wishes to Johnson but added: "It is important to have 100% clarity as to where responsibility for UK national security decisions now lies. We must anticipate adversaries attempting to exploit any perceived weakness."

Buckingham Palace and the Queen have been regularly updated on Johnson's condition, Downing Street said. The Prime Minister's two most senior officials, the cabinet secretary and his principal private secretary, will maintain contact with the palace on the Prime Minister's behalf, but the weekly audiences with the Queen will not continue for the time being.

The Queen has sent a message to Johnson's pregnant fiancée Carrie Symonds and the Johnson family. "Her Majesty said they were in her thoughts and that she wished the Prime Minister a full and speedy recovery," Buckingham Palace said.

The Duke of Cambridge also sent his best wishes. "Our thoughts are with the Prime Minister and his family, who like so many in the UK and around the world are affected by coronavirus," said Prince William.

A woman with a sign of support on her bicycle in London as leaders from across the world expressed their hope that Johnson would make a rapid recovery.

Johnson's spokesperson said that despite the lack of a formal succession procedure in the UK, there was an established order of ministerial precedence, with the top finance minister, Rishi Sunak following Raab. The "letters of last resort," sealed documents written by the Prime Minister to ballistic missile submarine commanders in case of a nuclear attack, still stand. "The Prime Minister remains the Prime Minister," the spokesperson said.

US President Donald Trump said at a Monday news conference that his administration had been in contact with Johnson's doctors. The Prime Minister's spokesperson said the government was "grateful for all of the warm wishes the PM has received overnight" and "confident he is receiving the best care from the NHS. Any treatment he receives is a matter for his doctors."

Gove told BBC Radio 4's Today program earlier that Johnson was "receiving the very best care" at St. Thomas' Hospital in London, after being taken into intensive care at 7 p.m. local time (2 p.m. ET) on Monday. Gove later said that he was self-isolating at home because a member of his family was displaying coronavirus symptoms. Gove said on Twitter that he did not have any symptoms and was continuing to work.

The coronavirus has hit the top of the British government hard. The Health Secretary, Matt Hancock, said on March 27 that he had the virus, on the same day that the chief medical officer, Chris Whitty, announced that he would begin self-isolation after displaying Covid-19 symptoms. Neil Ferguson, a top UK government adviser on the virus, said on March 18 he believed he had been infected.

Various other senior government ministers and advisers have been forced to self-isolate. Dominic Cummings, the Prime Minister's senior adviser, who self-isolated soon after Johnson's diagnosis, has not yet returned to work, Downing Street said.

Johnson was taken to hospital on Sunday evening. At the time, Downing Street said the decision was a precaution because he continued to suffer from a cough and a fever ten days after testing positive for the coronavirus. But his condition deteriorated on Monday, Downing Street said, and he was moved to the intensive care unit at St. Thomas' Hospital.

Gove told Sky News on Tuesday morning that Cabinet ministers were not told about the Prime Minister's deteriorating condition until nearly an hour after Johnson was taken into intensive care.

Asked whether the government had been up front with the public about Johnson's condition, and whether the Cabinet had been taken by surprise, he replied: "Yes we were. The [daily coronavirus] briefing that was given at 5 o'clock was given at a time when we didn't know about the deterioration in the Prime Minister's condition."

"We were informed subsequently. The Prime Minister was admitted to intensive care at 7 o'clock, and that information wasn't given to us in government -- to those in the cabinet -- until just before 8 o'clock."

Words of support

Politicians around the world sent best wishes for the Prime Minister's recovery.

Finance minister Sunak said on Twitter that his thoughts were with Johnson and his fiancée, Symonds. "I know he'll be getting the best care possible and will come out of this even stronger," he said.

Symonds has also experienced Covid-19 symptoms but said over the weekend she was "on the mend."

The Speaker of the UK's House of Commons, Lindsay Hoyle, said: "I know the thoughts and prayers of everyone across the House are with the Prime Minister and his family right now. We all wish him a speedy recovery."

Keir Starmer, the newly elected leader of the UK's main opposition Labour Party leader, tweeted: "Terribly sad news. All the country's thoughts are with the Prime Minister and his family during this incredibly difficult time."

Former Prime Minister Theresa May, who was replaced by Johnson in the country's Brexit crisis, wrote on Twitter said her "thoughts and prayers" were with Johnson and his family. "This horrific virus does not discriminate," she added.

French President Emmanuel Macron sent his "support to Boris Johnson, to his family and to the British people at this difficult moment," wishing him a "speedy recovery at this testing time."

Irish leader Leo Varadkar, who has returned to work as a doctor during the outbreak, tweeted that Johnson was "in our thoughts" on Monday night and wished him "a rapid return to health."

President Trump said on Monday: "We're very saddened to hear that he was taken into intensive care this a little while ago. He's been a really good friend. He's been really something very special. Strong. Resolute."

Clarification: This story has been updated to reflect Hoyle's role.