Three rowers, including a British competitor, were treated for apparent heat exhaustion and several others appeared to be unsteady on their feet during an Olympic test event in Tokyo at the weekend, underlining concern about the safety of athletes and spectators at next summer’s Games.

The British rower was stretchered to a medical station after he was unable to move following his race during Sunday’s world rowing junior championships at Sea Forest Waterway, the venue for the Tokyo 2020 rowing events.

Reports said the athlete, who has not been named, and two other participants required medical treatment after displaying symptoms of heatstroke. Kyodo news agency said several other athletes appeared to be “staggering” during the award ceremony.

Medical officials conceded that further measures would be needed in an attempt to reduce the effects of Tokyo’s blistering heat and humidity.

Kyodo quoted officials as saying that current measures, including ice baths and cooling mist sprays, were insufficient. “We will need to have a space with the proper equipment for athletes to cool off [after their events],” one said.

There were concerns, too, about the wellbeing of spectators at the same venue, with one reportedly treated for suspected heatstroke.

Cost-cutting measures imposed by the Tokyo metropolitan government mean that the grandstand’s roof covers only about half of the 2,000 seats, forcing many at the weekend’s test event to seek shade beneath parasols.

Kyodo quoted a Japan rowing association official as saying the roof needed to be extended to provide cover for all spectators. “The international rowing federation is also saying a roof is absolutely necessary, even if it is of the simplest design,” the official said.

One spectator said the high temperature, recorded at 33.7C just before 10am, had made him think twice about watching the Olympic rowing events in a year’s time. “I want to come watch the Olympic races, but this extreme heat makes me feel that my life could in danger,” he told the Yomiuri Shimbun newspaper.

The start time of Sunday’s men’s race in a marathon swimming test event at Tokyo’s Odaiba Marine Park was brought forward three hours to 7am, when the air temperature was already above 30C.

“That was the warmest race I’ve ever done,” three-time Olympic medalist Oussama Mellouli told Agence France-Presse after competing in the men’s 5km race. “It felt good for the first 2km then I got super overheated,” the Tunisian added.

Heatstroke was suspected in the death last week of a worker at an Olympic construction site, as Japan’s sweltering heat killed almost 60 people in the space of a week from the end of July.

The 50-year-old construction worker, who had been laying cable outside the Games’ media centre, died in hospital last Thursday after being found unconscious.

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

Olympic organisers have said they will introduce measures 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 have been moved to 6am as an extra precaution.