Two Metropolitan Police dogs - a German Shepherd puppy and a working Belgian Malinois - have died after being left in an unventilated private vehicle.
Officers, who forced their way into the vehicle after being alerted, found both dogs collapsed on Sunday.
They were taken from the dog training centre in south-east London to an emergency vets, where both later died.
An inquiry into the incident has begun and the Independent Police Complaints Commission has been made aware.
The RSPCA, also investigating the incident at Keston, near Bromley, where temperatures reached almost 30C on Sunday, advised people not to leave dogs in vehicles "for any length of time" during hot weather.'Tip of iceberg'
An RSPCA spokeswoman said: "Every year, the RSPCA receives about 6,000 calls from members of the public worried about dogs that have been left in cars during hot weather.
"But this may only be the tip of the iceberg and many cases may be unreported.
"Even when it is just 22C outside, the heat in a car can rise to 47C within 60 minutes," she said.
The RSPCA reminded dog owners that leaving a window open or a bowl of water in the car makes little difference.
The spokeswoman added: "All dogs will suffer although some are more prone to heatstroke than others. Those which are old, young, short-nosed, long-haired, overweight, heavily muscled or with certain health conditions are more at risk."