The prime minister has since been regularly photographed running in St James's park in Westminster alongside fitness expert Harry Jameson.
A Downing Street source confirmed to the Evening Standard that Mr Johnson had hired trainer to the stars, Harry Jameson, to help him get fit.
Saying: “Yes, he has engaged a personal trainer. I can confirm it is Harry Jameson. It’s because he is really serious about getting fit, as anyone who has seen him over the past couple of months knows, Boris is raring to go.”
Number 10 was forced to deny claims that Mr Johnson would be stepping down as prime minister because he was still dealing with ill-health related to his Covid-19 infection in April.
The father-in-law of Dominic Cummings was reported to have told a holidaymaker that Mr Johnson was still struggling with poor health after being admitted to intensive care earlier in the year. So-called 'long Covid' has been increasingly well documented by survivors.
But, who is the man that has been hired to help the prime minister?
Who is Harry Jameson?
Mr Jameson, who describes himself as an “elite performance coach” and “wellness business consultant” on Instagram, studied sports science at Liverpool University and has worked as a personal trainer for 15 years.
He also appears on Ms Whitmore's BBC radio show as the resident health wellness expert.
On his Instagram account, where he has more than 35,000 followers, the trainer regularly documents his boot camps in the Maldives and Turkey and also shares photos of his wife, Amy, and one-year-old son, Otis.
Mr Jameson is the son of Louise Jameson, an actor who in 1977-78 played Leela, a companion to Tom Baker’s Doctor, in Doctor Who.
She also played Rosa di Marco on EastEnders for more than 200 episodes.
What sort of exercise does Jameson do with clients?
Speaking to the BBC's Fit & Fearless podcast in July 2019, Mr Jameson, said he seeks to help his clients "in ways beyond pumping weights in the gym".
He says his approach has "always been based on a foundation of mind and body together and ultimately happiness".
As well as working as a fitness trainer, Mr Jameson owns Harry Jameson Consultancy – an organisation that offers “tailored and strategic advice” to businesses.
“Harry founded HJ Consultancy following a decade's experience in coaching individuals, companies and brands to achieve their potential,” the website reads.
“His acclaimed training philosophy considers the mind as much as the body, building on a dual specialism of Psychology and Sports Science to drive the best results from his clients.”
Why has Johnson hired Jameson?
Mr Johnson admitted that he had been “too fat” when he was admitted to hospital and spoke openly about his ambition to lose weight in a video to promote the government’s Better Health Strategy.
“I’ve always wanted to lose weight for ages and ages,” he said. “I think like many people, I struggle with my weight, I go up and down.”
Mr Johnson continued: “But since I recovered from coronavirus, I've been steadily building up my fitness.
“I don't want to make any excessive claims because I've only really just started concentrating on it, but I've got – I'm at least a stone down.”
Research has found that being obese doubles the risk of needing hospital treatment for coronavirus.