Heatstroke is suspected in the death of a worker at a Tokyo Olympics construction site, NHK national television has said, as the Japanese capital swelters through a scorching weather that has killed dozens of people.

Soaring temperatures have killed at least 57 people across Japan from 29 July to 4 August, highlighting the health threat to athletes and fans that is one of the biggest challenges for Olympics planners.

A 50-year-old construction worker who had been laying cable outside Tokyo Big Sight, which is being renovated to serve as the media centre for Tokyo 2020, was found unconscious on Thursday afternoon and taken to hospital, where he later died, NHK said.

Police were quoted as saying that conditions at the site suggested heatstroke may have been the cause, the broadcaster said, but police declined to confirm the report and Tokyo 2020 organisers were not immediately available for comment.

Temperatures in Tokyo have stayed above 31C (88F) since July 24, exactly a year before the Games are set to open, with the heat intensifying in August to average daily highs of 34.8C.

In addition to the 57 deaths since late July, 18,347 were taken to hospital. Some 45 people have died in Tokyo alone since 1 August, NHK said, many of them elderly residents living on their own.

Olympics organisers plan to take steps to deal with the heat, including shaded rest areas, tents at security checkpoints, mist sprays and ice packs.

The start times for the men’s and women’s marathons, on 9 and 2 August respectively, have been moved to 6.00am as an additional precaution.

Tokyo is hosting the Olympics for the second time, with the Games running from 24 July 24 to 9 August.