The charity says many people believe it is safe to leave their dog in the car if they're parked in the shade or the windows are open, but a car can become unbearably hot very quickly, even when the weather doesn't feel that warm.
Anne Wignall, trustee of RSPCA Shropshire branch, said: "When it's 22 degrees Celsius outside, the car could reach an unbearable 47 degrees within an hour. It's very dangerous and will cause your dog suffering and harm.
“It’s important never to leave your dog in a car on a warm day as it can quickly develop heatstroke. You can also help to save other people’s dogs. If you see a dog in distress in a hot car, it’s important to act straight away and dial 999 for the police.”
Signs of heatstroke include heavy panting, excessive drooling, lethargy, drowsiness, vomiting or collapse.
Anne said if the dog doesn't seem in distress, you can leave them in the car but it will be useful to work out how long they have been there.
"A 'pay and display' ticket could help. If you’re at a shop, venue, or event, ask staff to alert the owner over the loudspeaker. Make sure you, or someone else, stays with the dog and monitors their condition. If they get worse, be prepared to call 999."