The PM is being kept in intensive care at St Thomas' Hospital in London "for close monitoring", No 10 said.

Mr Johnson's condition is "stable" and he remains in "good spirits", his spokesman added on Tuesday evening.

Earlier, Foreign Secretary Dominic Raab said he was "confident" the PM would recover from this illness, describing him as a "fighter".

Mr Raab, who is deputising for the PM, said Mr Johnson was receiving standard oxygen treatment and was breathing without any assistance, such as mechanical ventilation or non-invasive respiratory support.

It is understood there will not be a further update on Mr Johnson's condition until later on Wednesday.

Downing Street also confirmed that the review into whether the coronavirus lockdown measures could be eased on Monday had been shelved.

Mr Johnson was originally admitted to St Thomas' on Sunday, on the advice of his doctor, after continuing to display symptoms of cough and high temperature 10 days after testing positive for the virus.

Speaking at Tuesday's Downing Street coronavirus briefing, Mr Raab said the prime minister was being monitored closely in critical care, as was usual clinical practice.

Describing Mr Johnson as not only a boss but "also a friend", Mr Raab said: "All of our thoughts and prayers are with the prime minister at this time, with Carrie, and with his whole family.

"And I'm confident he'll pull through, because if there's one thing I know about this prime minister, he's a fighter. And he'll be back at the helm, leading us through this crisis in short order."

Buckingham Palace said the Queen had sent a message to Mr Johnson's family and his pregnant fiancee, Carrie Symonds, saying she was thinking of them, and wished the PM a full and speedy recovery.

Prince William also tweeted a personal message of sympathy to the PM's family, signing it off with his initial "W", while his father, the Prince of Wales, sent a message from himself and the Duchess of Cornwall wishing Mr Johnson a "speedy recovery", Clarence House said.

'Very clear directions'

At the briefing, Mr Raab was also asked about whether his role deputising for Mr Johnson gave him full prime ministerial responsibility.

The foreign secretary said he was standing in for the prime minister "whenever necessary" - including leading the daily meetings of the coronavirus "war cabinet".

He said decisions would be made by "collective cabinet responsibility - so that is the same as before".

"But we've got very clear directions, very clear instructions from the prime minister, and we're focused with total unity and total resolve on implementing them so that when he's back, I hope in very short order, we will have made the progress that he would expect and that the country would expect," Mr Raab added.

Previously, Mr Johnson had committed to inspect the evidence to see if the lockdown measures could be eased in three weeks, which would be next Monday.

But Mr Raab said: "The critical thing is to take evidence-based decisions and so we've said that we will take any review once we've got the evidence that the measures are working.

"And having the kind of impact taking us past the peak which means that they can be responsibly done. We're not at that stage yet."

Downing Street confirmed that the review would not go ahead on the scheduled date and said it would instead take place after the three-week mark.

It came as the number of coronavirus hospital deaths in the UK rose to 6,159 on Tuesday - a record increase of 786 in a day, the Department of Health and Social Care said, compared with 439 on Monday.

However, the government's chief scientific adviser told the Downing Street briefing the number of coronavirus cases in the UK "could be moving in the right direction".

Sir Patrick Vallance said it was "possible that we're beginning to see... the curve flattening".

As of 09:00 BST on Tuesday, 213,181 people have been tested, of which 55,242 tested positive, the Department of Health and Social Care said.

Overall, 266,694 tests have been concluded, with 14,006 tests carried out on Monday.

In other developments:

A doctor who specialised in treating the elderly has died after testing positive for Covid-19
The first patients have been admitted to the Nightingale hospital in east London - a temporary facility set up at the Excel centre
Staff at Holland & Barrett are campaigning for their shops to be shut, arguing their health is at risk
The months-long lockdown in the city of Wuhan in China's Hubei province - where the coronavirus pandemic started - has been lifted
The number of people in France who have died from the coronavirus has now risen above 10,000.
New York recorded its highest single-day increase in virus deaths on Tuesday.
Cabinet Office minister Michael Gove became the latest politician to self-isolate after a family member developed coronavirus symptoms
People who volunteered to support the NHS in England during the coronavirus crisis are being given details of what tasks they can do to help