Boris Johnson is facing up to two months off work - regardless of whether he makes a full recovery - after being hospitalised with coronavirus, scientists have said.
They have warned the Prime Minister's stint in ITU at St Thomas' Hospital and subsequent recuperation period would lead to him losing substantial muscle mass and strength.
The PM will be left completely drained with his body expending all its energy fighting off the killer bug, the experts also warn.
Survivors of the virus have spent weeks in bed recuperating and Mr Johnson can expect a similar tough road to recovery.
Foreign Secretary Dominic Raab has been tasked with taking on much of the PM's role in the meantime and could be deputising for much longer than first anticipated.
The secretary of state confirmed on Tuesday that Britain had seen a further 786 deaths, moving the overall toll to 6,159, with cases rising by 3,634 to 55,242.
Downing Street has confirmed Mr Johnson, 55, is currently stable at the revered central London hospital and is no longer in need of ventilation - with Mr Raab calling him a "fighter".
The Queen, meanwhile, yesterday sent a heartfelt message to the PM's heavily pregnant fiancée Carrie Symonds - who is unable to visit Mr Johnson in hospital.
At a press briefing on Tuesday, Mr Raab said he was sure his boss will be "back at the helm leading us through this crisis in short order".
However, with the PM currently spending his second night in intensive care, one Tory source told the Daily Mail none of the party's members can "relax until he is out...and clearly on the mend".
Mr Johnson had ignored his doctor's advice by continuing to work long hours while isolating for 10 days from his Number 11 flat before being advised to go to hospital on Sunday as his condition deteriorated.
But Professor Paul Hunter, an expert in infectious diseases at the University of East Anglia, warned only around half of the patients who go into intensive care are surviving, adding: "Clearly you will need some time to recover."
"I would expect most people who were that ill, to need at least a month or possibly two to be sufficiently back and to be able to function," he told the Mail.
While Professor Mike Grocett, a consultant in critical care medicine at the University Hospital Southampton NHS Foundation Trust, said patients in the PM's situation "would have a decrease in physical function...that is likely to extend into weeks".
The vice president of the Royal College of Anaesthetists added: "We underestimate the value of just getting up and walking around and activity in normal life."
Matt Dockray, who was also treated in intensive care for the bug, has said there's "still a long road of recovery".
The dad-of-one, 39, from Marlow Buckinghamshire, told Good Morning Britain there was a point where he nearly "started to lose hope" after seeing the devastation in countries like Italy.
Experts are not clear yet on how long it takes someone struck by the virus to fully fight it off, with some reportedly believing symptoms can remain for two months.
Though Professor of Intensive Care Medicine at the University of Oxford, Duncan Young, said a patient who has been in ITU can be much longer than a month.
He said the average patient will remain on a normal hospital ward after leaving intensive care for around 15 days - and that's not Covid-19 related.
He added some people may stay as long as 48 days, saying it all depends on "what co-morbidities a patient has" as well as what their ailment is and how long they had to stay in ICU.
However, Dr Nathalie McDermott, a clinical lecturer at Kings College London, said she is a little baffled as to why the PM was even in intensive care if all he needed was standard oxygen therapy.
She said usually a patient in those circumstances would be in a High Dependency Unit, though admitted: "It's difficult to know."
Data from the Intensive Care National Audit and Research Centre - using outcomes of the first 2,621 patients admitted to intensive care units with coronavirus - has found 84 per cent who only needed basic respiratory support left the unit alive.
This is in contrast to 33 per cent who needed respiratory support, including ventilation.
Fewer than 35 per cent of those who needed ventilation on the first day of entering ITU pulled through, however.
While 50 per cent of those no longer needing care recover and 50 per cent die.
Some patients remain on the intensive care units for three weeks, but the average stay is just seven days, but experts have said Mr Johnson is more susceptible to complications due to his age and sex.