The Prime Minister has said he is sure Donald Trump is “going to be fine”, as the US president is treated for coronavirus in hospital.
Mr Trump posted a video on social media on Saturday in which he declared he felt “much better”, after conflicting reports over his condition.
Boris Johnson said the American leader is being given the “best” medical care available.
He told BBC One’s Andrew Marr Show: “I’m sure that President Trump is going to be fine, he has got the best possible care.
“The most important thing to do is follow his doctors’ advice.”
Mr Johnson has in the past admitted he himself could have died when he contracted coronavirus, which saw him admitted to intensive care in April with worsening symptoms.
Mr Trump is currently in the Walter Reed National Military Medical Centre near Washington DC, but he has told supporters he hopes to “be back soon”.
There have been conflicting reports about his health, with his physician Dr Sean Conley saying on Saturday that the president was fever-free, was not on oxygen and was not having difficulty breathing.
But a source familiar with Mr Trump’s condition told the Associated Press the president was administered supplemental oxygen at the White House on Friday before going to hospital, while White House chief of staff Mark Meadows briefed reporters that the Republican candidate was “not on a clear path to a full recovery”.
Mr Johnson has blamed his own weight for his close brush with Covid-19, declaring himself “too fat”.
During his interview on Sunday, he highlighted the risks posed by obesity when fighting coronavirus – although he insisted he was not commenting on Mr Trump’s weight.
Asked if contracting the virus should mean fewer cheeseburgers for Mr Trump, Mr Johnson replied: “This is an important point, obesity – I’m not making any comment on President Trump – but obesity, since you mention cheeseburgers, is one of the problems that this country needs to address.
“Not just because it threatens all our health but in the long term we need to tackle it to reduce the pressures on the NHS.”
Mr Trump’s Covid-19 diagnosis is the latest among world leaders, with Brazilian president Jair Bolsonaro and the EU’s chief Brexit negotiator Michel Barnier also having fallen ill.
Brexit Party leader Nigel Farage, an ally of Mr Trump’s, said he had spoken with his family on Friday following the diagnosis.
Mr Farage said, having watched the four-minute video of the billionaire speaking from hospital, he thought the president looked “robust”.
Speaking to Sky News’ Sophy Ridge On Sunday programme, he said: “He has got Covid-19, he has been ill, he’s been put into hospital, he’s been put on oxygen.
“But considering all of that, he looked pretty robust.”
He said it remains to be seen whether the president’s case of the virus will “bite hard or not”, but added: “He doesn’t drink, he doesn’t smoke, he’s carrying a few extra pounds but he is generally in pretty robust health, so my guess would be he is going to be OK.”
Meanwhile in the UK, almost 13,000 new cases of Covid-19 have been confirmed – but the Government said a technical glitch had led to previous under-reporting of figures.
The Government said that as of 9am on Saturday, there had been a further 12,872 lab-confirmed cases of coronavirus – almost double the previous day when a further 6,968 infections were reported.
The error means the total reported over the coming days will include some additional cases from the period between September 24 and October 1.
Northern Ireland Secretary Brandon Lewis said the revisions were evidence of the Government and NHS Test and Trace being “transparent” about the testing numbers.
He told Sophy Ridge: “As soon as they’ve spotted there was an issue, they’ve dug into that, they’ve got to the bottom of those numbers, they have been transparent and published the correct numbers and of course the teams will be looking through that to ensure it doesn’t happen again.”
In a fierce criticism of the Government’s recent performance, Labour leader Sir Keir Starmer accused Mr Johnson of having “lost control” of the fight against the virus and having no clear strategy.
One in three people in the country are currently living under heightened lockdown restrictions as the Government fights to control the disease.
Sir Keir said Mr Johnson should produce a new “road map” for navigating the country through the winter and has offered his own five-point plan for getting the country to a vaccine.
Mr Johnson warned on Sunday that while he hopes for a turnaround by Christmas, there could be “bumpy months ahead”.
He said: “This could be a very tough winter for all of us – we’ve got to face that fact.”