While the photo is one likely to appear in many family albums, William and Catherine have faced criticism from some animal cruelty campaigners who ridiculed them for giving the spaniel treats.
Experts warn that dogs are allergic to both dairy and chocolate and feeding them milk-based products such as ice cream could cause them digestive issues.
On Twitter, one person took extreme offence to the photographs and branded the prince a "monarchist monster".
They added: "Never give a dog ice cream. At worst: it'll die, At best: the dog will diarrhoea everywhere!"
The offending ice cream. Photo / Supplied
Another even remarked, rather dramatically, that the youngster should "go to prison" for giving the animal what appeared to be flavoured ice cream with a white chocolate coating.
Adult dogs are deficient in lactose necessary for digestion of milk, one animal charity warned.
The royal family released four new images of George to coincide with his third birthday. Photo / Supplied
Chocolate is also harmful to pets as it contains theobromine which is toxic to animals. The chemical is found in all flavours of chocolate, including white.
Consumption of chocolate can result in abnormal heart rhythm, tremors, seizures, and death in dogs. Less severe symptoms include vomiting, diarrhoea and excessive dehydration.
A spokesman for the Royal SPCA told the Daily Mail: "'It is lovely that Prince George is trying to help keep his family dog, Lupo, cool in these high temperatures.
"[But] we would advise people to be cautious when giving their dogs food meant for human consumption as some items, like chocolate, can be highly toxic to dogs and dairy items can be difficult for them to digest.
Prince George on the swing at his parents' country home, Anmer Hall. Photo / Supplied
"Instead of ice cream we would suggest making an ice lolly from pet-friendly ingredients. Making these can be really fun for children and the end product is both safe and enjoyable for dogs."
Paula Boyden, veterinary director of the Dogs Trust charity, added: "We would always advocate seeking methods to keep your dog cool, but possibly not in the form of an ice cream, unless it is a dog-friendly version.
"The cow's milk and sugar content in ice cream can sometimes lead to digestive issues. Whenever you do treat your dog, do consider the ingredients and always seeks the advice of your vet."
One of the four official portraits of Prince George released for his birthday. Photo / Supplied
The image of Prince George appearing to feed Lupo ice cream emerged in a touching collection released to mark his birthday.
The little prince, third in line to the throne, can be seen in two shots playing on a swing, at his parents' country home, Anmer Hall, that is inscribed with their names, William and Catherine. It is believed to have been a wedding gift to the couple.
Foods your dog should avoid
Dairy: Dogs are lactose intolerant which means they are allergic to milk and milk-based products such as ice cream, cheese and chocolate. It can cause them vomiting, diarrhoea and itchiness.
Chocolate: Dogs can suffer digestive issues, tremors and seizures if they consume chocolate as it contains theobromine, which they are allergic to. Most will experience vomiting and diarrhoea but consumption of chocolate can result in death for dogs.
Alcohol: Alcohol has the same effect on a dog's liver and brain that it has on humans. But it takes far less to do its damage so they could end up in a coma or even die after consuming a small amount. They are also likely to suffer vomiting, diarrhoea, central nervous system depression, co-ordination problems and breathing difficulties.
Avocado: Avocados contain persin which is toxic for dogs in large quantities. Onions and garlic - These foods in all forms - including powdered, raw, cooked, or dehydrated - can destroy a dog's red blood cells and cause anaemia.
Caffeine: Tea, coffee and other caffeine-related products can be fatal to dogs because they poison their systems. Caffeine can also cause them restlessness, rapid breathing, heart palpitations, muscle tremors, and fits.
Grapes and raisins: These can cause kidney failure in dogs. Nuts should also be avoided.
Sweets: Candy, gum, toothpaste, baked goods, and some diet foods are sweetened with xylitol which causes more insulin to circulate through a dog's body. That can cause blood sugar to drop and result in liver failure.
Fat trimmings: Fat trimmed from meat, both cooked and uncooked, can cause pancreatitis.
Peaches, and Plums: The problem is the seeds or pits which can cause inflammation of the small intestine in dogs.
Raw eggs: This can give a dog food poisoning from bacteria like Salmonella or E. coli. Raw eggs also contain an enzyme that interferes with the absorption of a particular B vitamin.
Salt: Eating too much salt can cause excessive thirst and urination and lead to sodium ion poisoning. Symptoms of too much salt in dogs include vomiting, diarrhoea, depression, tremors, elevated body temperature and seizures. It may even cause death.
