Dr Hilary Jones has issued a new warning amid the possibility of gyms reopening in the UK.
Gyms and leisure centres were forced to close in March due to the coronavirus outbreak, but Prime Minister Boris Johnson could soon announce plans to reopen fitness facilities.
Mr Johnson told LBC radio on July 3: "We are going to reopen gyms as soon as soon as we can do it in a Covid secure way and I think that the date for reopening gyms at the moment, if we can do it, is in just a couple of weeks time."
But Dr Hilary has issued a stark warning on Good Morning Britain over high transmission fears.
Speaking to Ben Shephard and Ranvir Singh, he said: "The thing is, if you're in a gym, and we've all been to gyms, if you're doing aerobic work then you're breathing very fast.
"Anything that's in your nose and throat is being expelled much more explosively than if you're sitting quietly in a pub."
After Ben pointed out that there would be strict protocols in place, Dr Hilary replied: "Sure, but how much do we trust those protocols?
"We owe a huge debt to the brilliant cleaners in the last few months that have done a sterling job keeping everything as clean as possible.
"We are absolutely reliant on every piece of equipment in the gym being cleaned."
He added: "If you can open the windows and if you can get ventilation in there and you can separate the spin bikes. You don't want 30 people on spin bikes where you can touch the person next to you. Breathing, spluttering. That's not what you want."
"It is about common sense and about hygiene and really strict protocols, I agree," he continued. "For some people, they will say 'I'm going to take a sensible decision' and they will go for a run down the street or in a park and that is going to be much safer than going to an indoor gym."
Good Morning Britain continues weekdays on ITV at 6am.
