During a two-day heatwave in July, officers from the RSPCA were called to 106 reports of pets left in cars, conservatories and caravans.
In three of the call-outs, rescue teams arrived too late and the animals all died as temperatures rose to unbearable levels.
Making an emotional statement, RSPCA inspector Justin Stubbs described the devastating toll that witnessing the events had taken on him.
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“To witness a dead dog in a hot car is particularly harrowing,” he said. “When you go home at night, it is hard to put out of your mind that they would have slowly stifled to death, gasping for air.
“When a dog is left in a car for a prolonged period of time their skin blisters and they suffocate. It’s hard to think of anything worse.”
Police are now said to be investigating the incidents, which all took place in Peterborough.
But as temperatures look set to remain high throughout August, the RSPCA has renewed its calls for drivers to be aware of the fatal effects heat can have on dogs.
The animal charity says fatalities can occur as a result of overheating, even on days when it might not feel particularly warm. It highlights that temperatures inside vehicles can quickly rise above those outside in as short a time as an hour.
“There is no good reason to bring your dog out in your car in warm weather and leave it while you go to do your shopping, doctor's appointment, or visit the pub,” Inspector Stubbs said.
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In separate statistics, the RSPCA revealed that it has seen a spike in call-outs relating to dogs in hot cars.
It received 3,000 more calls concerning heat exposure in dogs last year compared to five years ago.
According to the figures, there was a peak in 2014, when the RSPCA had to deal with 10,229 reports of such incidents.
Last month, dog lover Jess Ritchie filmed herself overheating in a car in a bid to raise awareness of the dangers of high temperatures.
After sharing the footage on social media, it was viewed more than 6,000 times right around the world.