Boris Johnson insisted Donald Trump is 'going to be fine' today as he dismissed 'balderdash' rumours about his own coronavirus scare.
The PM said the President had been getting the 'best possible care' and he was 'sure' he will overcome the disease.
Speaking on the BBC's Andrew Marr Show, Mr Johnson also batted away speculation that he has not fully recovered after having the disease in April.
He flatly denied suffering 'long Covid', joking that he was 'fitter than several butchers' dogs'.
And the premier bluntly blamed his own weight for having to go to hospital - although he stressed he was not making a point about Mr Trump's condition. 'I was too fat,' Mr Johnson said.
The comments came after Mr Trump said he was feeling 'much better', despite conflicting reports over his condition.
The president's medical team said he was doing 'very well' and was in 'exceptionally good spirits' in an update yesterday, less than 24 hours after he was taken by helicopter to the Walter Reed National Military Medical Centre near Washington DC.
However, White House chief of staff Mark Meadows briefed reporters soon afterwards that Mr Trump had gone through a 'very concerning' period on Friday and the next 48 hours would be critical.
The president himself then posted a video on Twitter from his hospital suite, saying he was feeling 'much better now' and hoped to 'be back soon'.
Boris Johnson (pictured arriving at the BBC studios today) said the President had been getting the 'best possible care' and he was 'sure' he will overcome the disease
Mr Trump posted a Twitter video yesterday saying that he was 'starting to feel good'
'The most important thing to do is follow his doctors' advice.'
Asked if that should mean fewer cheeseburgers for the president, Mr Johnson said: '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.'
Pressed on persistent rumours that he himself is still struggling to shake off the after-effects of the disease - known as 'Long Covid' - Mr Johnson said: 'No, no, not in my case.
'This is total tittle tattle, it is drivel. It is not tittle tattle, it is balderdash and nonsense.
'I can tell you I'm fitter than several butchers' dogs.'
He went on: 'The issue is when I got this wretched thing I was, alas, too fat.
'If I may say so, this is a teachable moment for our great country because we are one of the greatest places on Earth but, alas, as a nation we are slightly too fat.'
Nigel Farage, an ally of Mr Trump, said he thought the US president looked 'robust' in his video from hospital which was shared on social media.
The Brexit Party leader told Sky News' Sophy Ridge On Sunday programme: '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 there was still a wait to find out whether his virus would '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.'
Mr Farage said the president's illness was likely to be 'bad for his campaign' for re-election in the 'short-term' but not a terminal factor.
'It is bad because it puts Covid at the centre, it is bad because the accusations are he was too lackadaisical and it is bad because he can't do what he is good at which is rallies, campaigns and rallying the troops which he really is brilliant at,' said Mr Farage.
Sir Christopher Meyer, the UK's former ambassador to the US, said Mr Trump's explanation for going to hospital was 'astonishing'.
Speaking to the Sophy Ridge On Sunday show on Sky News, Sir Christopher said: 'I don't think they're being terribly open at all.
'You're right about conflicting messages. You've had one apparently from the White House chief of staff Mark Meadows giving a run-down to the White House pool of reporters which was much more pessimistic than anything that Donald Trump himself was saying or even his personal doctor Sean Conley who came out with a bunch of white-coated people today.
'So there's a good deal of scepticism about what his private doctor is saying and what he, Trump, himself is saying.'
Responding to a video posted to Mr Trump's Twitter account showing him in hospital, Sir Christopher said: 'I'm astonished at the reason he has given... it's pretty flagrant even by Trump's standards, that the reason he went to hospital and didn't stay in the White House was because he 'wanted to join the American people and share their suffering and show leadership'.sonos sonos One (Gen 2) - Voice Controlled Smart Speaker with Amazon Alexa Built-in - Black read more
'This is rubbish. The reason he is in hospital is because he needs to for medical reasons.'
White House physician Dr Sean Conley sent a letter to press secretary Kayleigh McEnany saying that the president and first lady were 'well' and promising to keep