* PM has received oxygen but is still conscious
* Foreign minister Raab stands in for him
* Queen Elizabeth wishes PM Johnson a speedy recovery (Adds latest death number)
LONDON, April 7 (Reuters) - British Prime Minister Boris Johnson was stable in intensive care on Tuesday after receiving oxygen support to help him battle COVID-19, while his foreign minister led the government's response to the outbreak.
Johnson's personal battle with the virus has shaken the government just as the United Kingdom, now in its third week of virtual lockdown, enters what scientists say will be the most deadly phase of the coronavirus pandemic that has already killed at least 6,159 people in the country.
Johnson, 55, was admitted to St Thomas' Hospital across the River Thames from parliament late on Sunday after suffering persistent coronavirus symptoms, including a high temperature and a cough, for more than 10 days.
But his condition rapidly deteriorated over the next 24 hours, and he was moved on Monday to an intensive care unit, where the most serious cases are treated, in case he needed to be put on a ventilator. He was still conscious, his office said.
"He is receiving standard oxygen treatment and breathing without any other assistance," Johnson's spokesman told reporters at an 1100 GMT briefing.
"The prime minister has been stable overnight and remains in good spirits," the spokesman said. "He has not required mechanical ventilation, or non-invasive respiratory support."
But the absence of Johnson, the first leader of a major power to be hospitalised after testing positive for the novel coronavirus, has raised questions about who is truly in charge of the world's fifth largest economy at such a crucial time.
While Britain has no formal succession plan should a prime minister become incapacitated, Johnson asked Foreign Secretary Dominic Raab, 46, to deputise for him "where necessary", Downing Street said. If Raab is incapacitated, finance minister Rishi Sunak would take on those responsibilities.
Queen Elizabeth wished Johnson a "full and speedy recovery" and sent a message of support to his pregnant fiancée, Carrie Symonds, and his family. Prince Charles, the heir to the British throne, wished Johnson a speedy recovery.
Raab chaired the government's COVID-19 emergency response meeting on Tuesday, though ministers refused to say who had ultimate control over the United Kingdom's nuclear weapons - a role held by the prime minister.
"There are well-developed protocols which are in place," said Cabinet Office Minister Michael Gove, who himself went into self-isolation on Tuesday after a family member displayed coronavirus symptoms.
British leaders do not traditionally publicise the results of their medical examinations, as some U.S. presidents including Donald Trump have.
Raab, the son of a Czech-born Jewish refugee who fled the Nazis in 1938, takes the helm at a pivotal time. Government scientists see the death toll rising until at least April 12 and Britain must ultimately decide when to lift the lockdown.
Johnson's move to intensive care added to the sense of upheaval that the coronavirus has wrought as its spread has caused global panic, sowed chaos through financial markets and prompted the virtual shutdown of the world economy.
The United Kingdom is in a state of almost total lockdown, a situation due to be reviewed early next week. Some ministers have suggested the lockdown might need to be extended, with evidence that some people were using sunny spring weather as an excuse to flout the strict rules.
The death toll announced on Tuesday had risen by 786 - the biggest daily increase to date.
The pound dipped in Asian trading on news of Johnson's intensive care treatment but then rallied in London trading. Against the dollar, sterling traded to a high of $1.2349, up 0.9% on the session.
Even before coronavirus, Johnson had had a tumultuous year.
He won the top job in July 2019, renegotiated a Brexit deal with the European Union, fought a snap election in December which he won resoundingly and then led the United Kingdom out of the European Union on Jan 31 - promising to seal a trade deal with the EU by the end of this year.
The government has said it is not planning to seek an extension to that deadline in light of the epidemic.
Johnson has faced criticism for initially approving a much more modest response to the coronavirus outbreak than other major European leaders, though he then imposed a lockdown as projections showed half a million people could die.
He tested positive for the virus on March 26.
After 10 days of isolation in an apartment at Downing Street, he was admitted to hospital. He was last seen in a video message posted on Twitter on Friday when he looked weary.
- UK PM Johnson 'stable' in intensive care, needed oxygen after COVID-19 symptoms worsened (Reuters: Apr 7, 2020 at 2:30 PM)