Ambulance crews had to help more than 50 runners who struggled with the heat during Sunday’s Dam to Dam race.

The 10-mile (16km) race from Amsterdam to Zaandam was abandoned at 3.30pm, with the last two groups of 4,500 runners still waiting to start, as temperatures reached 25C on a course with little shade.

Race director René Wit told local news station AT5 that there was no choice but to halt the race after dozens of runners had to be treated for heatstroke and other conditions.

“We know it was going to be a warm day with lots of sunshine and took precautionary measures, but it was even warmer,” he said. “When it gets to the point that the medical services can’t cope with the pressure, you have to take decisions.”

Organisers will contact the runners who received medical help to see if their fitness was an issue. “We don’t know exactly who was affected, how old they were or how well trained they were,” Wit said.

Wit said he had noticed increasing numbers of runners in recent years taking part without proper training, but denied that the event had grown too large. Around 80,000 people took part in the Damloop and the Dam to Dam race over the weekend.

Wit said organisers would consider measures to ease the strain next year, such as spreading out the start times over a longer period.

In May this year the Leiden marathon was abandoned after 25 runners were taken to hospital suffering from heat exhaustion.