Even in temperatures of 20°c, dogs are at risk of heatstroke. I see so many people walking their dogs in this heat, and even higher temperatures. These dogs are usually walking slowly behind the owner; excessively panting, trying to stop, etc. whilst the owner continues to exercise them.  

There are many infographics, articles, and vets advice stating the exact temperatures that are too hot. Below is a list of the temperatures, and the level of risk just to put it into perspective. 

12-15°c : No evidence of heatstroke risk. 

16-19°c : Generally safe to exercise dogs at all times of day. Although keep an eye on overweight dogs, large breeds, and brachycephalic breeds. 

20-23°c : Dogs are at risk of getting heatstroke if exercised too rigorously, or suffering from underlying health conditions.

24-27°c : Extreme caution should be taken as most dogs, especially those that are overweight, large, very young or brachycephalic, will find these temperatures very uncomfortable.

28-31°c : Dangerous for all, but life threatening for elderly dogs, puppies, large breeds, overweight dogs & brachycephalic breeds. 

32°c + : Heat stroke is a major risk, regardless of size, age or breed. 

As you can see from the information provided, at 25°c, dogs are massively at risk. For this reason, I believe fines should be given to owners exercising their dogs at these temperatures.