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The owner reportedly left a 14-week-old terrier mix in the car while doing some shopping, with police in Swindon, Wiltshire, called to reports of a dog in distress at around 10am on Monday.
Witnesses said a member of the public - who has remained unnamed - smashed the window to let the dog out and police subsequently deemed his actions lawful.
Pictures show attending community support officers looking after the dog before giving it back it to the owner upon his return, with police and a representative from the RSPCA issuing the owner with a formal warning over the incident.
Speaking at the scene, Inspector Steph Daly, of Wiltshire Police, said: "People just don't think it's going to happen to them.
"They think 'oh I'll be alright, I'm only going to be five minutes', but with what's going on in the world at the moment, we have to queue for the supermarkets. You know your trip is going to take longer.
"People are animal lovers, and if they see something they're not happy with, they'll take action."
Inspector Daly added that while few of us would ever deliberately do harm to our own pets, recklessness or ignorance can lead owners to commit an offence and jeopardise the safety of their pets.
He said: "At the end of the day, the RSPCA advise people not to leave their dog in the car full stop. It might be 20 degrees outside, but it could be up to 38-40 degrees inside the car.
"What I say to people is: sit in a car yourself for five or ten minutes with no windows open and no air conditioning on and see how long you can last.
"We can sweat though, dogs can't. If you take that risk of leaving your dog in a car, then to me you're being reckless.
"You're not deliberately trying to hurt your animal, but that's still committing an offence."
While many of us may instinctively think to smash a car window if we see a dog in distress inside, it is worth noting that it could be classed as criminal damage without proper justification.
If the dog is displaying signs of heatstroke, such as drooling or panting excessively, people are advised to call 999. If the situation becomes critical and police have not arrived, you should take photos or videos as evidence and take details of any witnesses before smashing a window.
The law states that you have a lawful excuse to commit damage if you believe the owner of the property that you damage would consent to the damage if they knew the circumstances.
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