Police officers in Harrow had to force their way into a van to rescue a dog left for two hours with no water on a day where temperatures reached a high of 32C.
A concerned passer-by called 999 at 12.30pm on Friday after noticing the Rottweiler, named Toby, had been left alone on Carlton Avenue for some time.
The owner was issued with a summons, while Toby was taken to the vet for a check up.
A Harrow Police spokesman said: "Our officers found the dog in quite a distressed state, he was panting and could not breathe well as there was no air.
"Luckily, there was a gap so we were able to force the window open, get the dog out and put a wet blanket over him.
"The vet said he appeared fine but if it had been another hour or so it would not have been such a happy ending."
Toby was then taken to police kennels, where he was hosed down and given water and treats. He will be assessed and looked after at a more central police kennels until a court determines what should be done with him.
The owner returned while officers were still on the scene and will be prosecuted.
The spokesman added: "I think he realised what he had done but did not realise how serious it was, the dog did not otherwise look neglected.
"We want people to be aware that they cannot leave dogs like this and that if they see an animal locked in a hot car, they should phone 999."