France won a thrilling mixed triathlon test event for the ­Tokyo 2020 Olympics Sunday, which passed off as planned in a welcome boost for organizers after difficulties with high temperatures and water pollution.

The swimming leg of the paratriathlon on Saturday was scrapped after water tests showed levels of e-coli bacteria more than double the acceptable standard and the running section of a women's test event on Thursday was curtailed in blistering temperatures, with French athlete Cassandre Beaugrand treated for heatstroke.

But on Sunday in the Japanese capital with the temperature a slightly cooler 30 C, the levels of bacteria reduced to within permitted levels.

The mixed relay, a new event for Tokyo 2020 involving two men and two women, was won by France in a photo finish from Great Britain after a nail-biting final sprint.

Dorian Connix chased down Alex Yee on the final straight despite being behind for most of the race and breasted the tape fractionally ahead of his British competitor. A powerful US team came third.

The format is much ­shorter than the individual race with each athlete completing a 300-meter swim, a 7.4-­kilometer cycle and a two-kilometer run before handing off to a teammate, making it less demanding in hot weather.

Battling the heat of the sweltering summer has become the main challenge for organizers, who have otherwise won widespread praise, including from the IOC, for their preparations.

Yasuo Mori, deputy ­executive director of the Tokyo 2020 Games Operations Bureau, said athletes needed to have greater access to water during the race.

The International Triathlon Union (ITU) would examine increasing the number of water supply points and was deploying medical staff every 500 meters along the course in case of an emergency, Mori added.

Aside from the problems at the triathlon, about a dozen people, including a spectator, were taken ill at a rowing test event in the previous week with suspected heatstroke.

A construction worker also died on an Olympic site as ­Tokyo sweats through a prolonged and deadly heat wave.

The triathlon test events have courted controversy even without the heat - two British athletes were disqualified on Thursday for crossing the line first holding hands.