Tokyo 2020 advisor Makoto Yokohari has claimed heat may be a "nightmare" at next year's Olympic and Paralympic Games.
Doubts still swirl over the likelihood of Tokyo 2020 taking place in 2021, with cases of coronavirus still high in various countries around the world.
Yokohari, a professor of environment and urban planning at the University of Tokyo, suggested that even if the Games were held, they would be marred by heat and humidity.
He analysed data going back to the Los Angeles 1984 Olympics, and found Tokyo had the highest average temperature and precipitation of a host city during the period when the Games were held.
Temperatures this July, when the Games were originally due to take place, were significantly cooler in the Japanese capital then on average, however.
"When it comes to heat stress or heat stroke, the problem is not only the temperature but also the humidity as well," Yokohari said, as reported by Reuters.
"When you can combine these two... Tokyo is the worst in history."
"The temperature [this year] was not that high so if we could have this kind of weather next year, maybe it will be okay.
"But, if the weather turns out like right now, or like last year, then it is going to be a nightmare."
Heat became a major concern for Tokyo 2020 after more than 50 deaths were recorded in Japan because of rising temperatures in July last year.
A number of heat-prevention measures were introduced, including the use of real and fake snow, cooling mist sprays and shading tents.
Fears also prompted organisers to controversially switch the marathons and race walking to Sapporo, more than 800 kilometres to the north of Tokyo.
Concerns about heat were soon overshadowed by the coronavirus pandemic, however, with the global health crisis forcing the postponement of Tokyo 2020 to 2021.
The Olympics are now scheduled for July 23 to August 8 next year, followed by the Paralympics from August 24 to September 5.