BORIS Johnson is back on his feet and has been able to do "short walks," No 10 has said.

In the latest update on his recovery from coronavirus, A Downing St spokesman said: “The Prime Minister has been able to do short walks, between periods of rest, as part of the care he is receiving to aid his recovery.

“He has spoken to his doctors and thanks the whole clinical team for the incredible care he has received.

“His thoughts are with those affected by this terrible disease.”

After spending his fifth night at St Thomas’s Hospital in London, Mr Johnson is now in a general ward after being moved out of the intensive care unit, where he spent the best part of three days.

His spokesman revealed that Mr Johnson had "waved his thanks" to staff as he was moved to the general ward, stressing how he was "enormously grateful" for the care they had given him.

He emphasised how the PM was in "very good spirits".

Consultant virologist Dr Chris Smith, from the University of Cambridge, who is also presenter of the Naked Scientists podcast, said when the PM was fit enough to be discharged from hospital, it was likely he would do so with instructions to take it easy for several weeks.

He explained becoming ill enough to warrant a stay in an intensive care unit left a patient “weak and exhausted” for a significant period of time.

Mr Johnson was likely to be feeling "like he's been hit by several buses," declared Dr Smith.

"Being severely unwell and sufficiently ill to warrant ITU admission really takes it out of you and leaves people weak and exhausted for a significant period of time afterwards."

However, as the PM was younger – he is 55 - was in good health, and had not needed to be put on a ventilator, "he is likely to be able to bounce back more quickly".

Dr Smith explained: "It'll still be a few weeks before he's feeling like his old self though."

He claimed doctors and nurses would be aiming to get Mr Johnson home as soon as possible.

"To make this decision they'll monitor respiratory function and other vital signs and, if he's stable and continuing to improve - no evidence of ongoing infection, or other secondary infections - and not oxygen-dependent, including at night, then he'll go home with instructions to take it easy for - at least - several weeks to recover."

Dr Smith added that it took at least a week to recover for every day that someone had been in intensive care – which in the PM’s case would be around three to four weeks - and advice to patients includes staying active, eating and drinking well, and building up strength gradually.

Earlier, the PM's spokesman said: "I am told he was waving his thanks towards the nurses and doctors that he saw as he was being moved from the intensive care unit back to the ward," the spokesman explained.

"Hopefully, it was clear to the staff that he was waving his gratitude."

He said he was not aware of any contact between Mr Johnson and No 10 and that it was too early to say how long he would need to remain in hospital.

"The Prime Minister is back on a ward and continuing his recovery which is at an early stage. He continues to be in very good spirits," the spokesman said.

Addressing when Mr Johnson might return to his No 10 desk, he added: "Decisions such as this will be on the advice of his medical team. They have given him brilliant care."

Meanwhile, Mr Johnson Snr said his son's illness had underlined the seriousness of the pandemic.

"To use that American expression, he almost took one for the team. We have got to make sure we play the game properly now. This is pretty straightforward now. He must rest up.”

He went on: “He has moved from the ICU into a recovery unit but I don't think you can say this is out of the woods now. He has to take time. I cannot believe you can walk away from this and get straight back to Downing Street and pick up the reins without a period of readjustment."

The PM’s father said the whole family was "amazingly grateful" for the efforts of the NHS and for the huge outpouring of support for his son.

Another expert, Professor Duncan Young, an ICU consultant, said staff at St Thomas's would be monitoring the PM's need for oxygen and assessing when he would be able to go home.

"My guess is he is now on a normal oxygen face mask; that's the point when it is likely someone is discharged from ICU.

"As his need for additional oxygen goes down, the hospital staff will dial down the level of oxygen, and it will get to the point where he can breathe ordinary air. Staff will have to decide at that point whether it is safe for him to go home."

He explained as well as being able to breathe without added oxygen, doctors and nurses would be looking to see if Mr Johnson was able to do things for himself, such as walk and eat.

Once he was discharged and was able to go home, Mr Johnson would still be likely to experience shortness of breath and lethargy, said Prof Young.

"He has been very ill and it will take a while," he stressed, noting: "Nobody knows in terms of shortness of breath and lethargy, in scientific literature, how long it takes to recover. It particularly depends on how ill you have been.

"Sitting in a chair is one thing, getting up and doing things puts more pressure on your system."

The likelihood is that Dominic Raab, the Foreign Secretary, will continue to deputise for the PM for some time yet.

As well as his position as Foreign Secretary, Mr Raab is First Secretary of State and it was in this role that Mr Johnson asked him to stand in for him after his symptoms worsened on Monday afternoon.

In addition to leading the Government's daily coronavirus "war cabinet" sessions, Mr Raab will also chair any necessary meetings of the National Security Council in Mr Johnson's stead, it has previously been confirmed.

Meanwhile, as the Government launched a social media campaign urging people not to socialise over the long Easter weekend, Stephen Powis, NHS England's National Medical Director, said it was "critical" people continued to obey the social distancing measures to reduce the spread of coronavirus.

“It is really critical this weekend that we keep obeying those instructions. We are beginning to see the benefits of this social distancing. We do believe the virus is spreading less. That will only continue to happen if we don't get complacent and continue to follow those instructions."

Mr Powis stressed it was "still too early" to say whether the UK had reached the peak of its coronavirus cases.

“It's really difficult to predict where any peak of that plateau will be,” he told BBC Breakkfast.

"There is a whole host of things we want to look at, we want to see deaths falling. Unfortunately, for the next week or so they will continue to be high because it is the very last thing that changes when you introduce these measures.”

But Mr Powis warned: "It's still too early to really be confident that we are turning the corner. We need to completely and utterly make sure that we all comply with the instructions we have been given."

However, he gave some hope that the lockdown restrictions could begin to be eased next month.

He explained social distancing measures would be eased when the number of coronavirus-related deaths started to fall, which could be in "one or two" weeks’ time.

Asked what it would take for the current social distancing measures to be relaxed, he said: "Clearly, we want to see deaths falling, and that could be a week or two yet.

"We want to see pressure on the NHS not building any more, we want to see fewer people on intensive care units and admitted to hospital with the virus.

"We want to see evidence that the virus is not spreading as much in the community."

Leading epidemiologist and Government advisor, Professor Neil Ferguson of Imperial College London appeared more cautious, suggesting the lockdown restrictions would have to remain in place for "several more weeks".

He explained the social distancing measures appeared to be working better than expected but the experts would need to see more evidence the spread of the disease was being reduced.

"We made quite conservative assumptions about the level of contact reduction these measures would result in. There is some preliminary evidence in terms of contact surveys, in terms of data from companies like Google about how people move, that we have seen even larger reductions in normal behaviour, contact, than we would have dared hope.

"That is good news but we have still got to see that reflected in case numbers coming down. It is only when we see the case numbers come down and how quickly transmissions have been reduced, we can really conclude anything about what happens next, when these measures can be relaxed."

Prof Ferguson also said the restrictions could be lifted in stages and he pointed out there would need to be more testing for cases of the disease.

"Without doubt measures will be targeted, probably by age, by geography, and we will need to introduce much larger levels of testing at a community level to really be able to isolate cases and more effectively identify where transmission has happened," he added.

Professor Paul Cosford, Medical Director for Public Health England, said it was "not unreasonable" to expect the lockdown to continue for several weeks.

"Several weeks isn't unreasonable; let's hope it’s sooner than that," he told BBC Radio 4's Today programme.

"All my experience dealing with any sort of infectious disease suggests once you start getting things under control, that is the time you absolutely need to continue with all your measures so you can bring the disease right down, essentially to crack it across the country."

He too suggested the restrictions could be lifted in stages.

"I could conceive of circumstances in which some of the restrictions are lifted sooner and some are lifted later. Just for now, we are getting on top of this but we have got an awfully long way to go and it is absolutely critical that we continue with all the actions that are required of us," he added.