Leaving dogs in parked cars can be potentially dangerous all year round, even in the winter when outside temperatures are low, researchers have warned.

A study by experts in dog welfare at Nottingham Trent University has found temperatures inside cars are hot enough throughout the year to pose a risk to dog health.

The researchers monitored internal temperatures of cars in the UK, which had no dogs inside, every day for two years.

They found temperatures exceeded 25C in every month of the year, high enough to cause overheating in breeds with flat faces, such as bulldogs and pugs.

Most dogs are comfortable at temperatures between 15-25C, but this is dependent on breed, coat length, fitness and a range of other factors, the researchers said.

People assume the risk is only midday during the summer, when in fact cars can reach potentially dangerous temperatures all year round, with late afternoon the hottest time period

The team also found the highest internal temperatures in vehicles occurred between 4-5pm, and exceeded 35C between April and September.

Dogs need to pant to control their body heat if temperatures exceed 35C.

However, in enclosed vehicles, panting can be harder for dogs due to humidity and lack of air movement, resulting in reduced latent heat exchange.

Along with panting, signs of heat stoke in dogs include red or dark gums and tongue, confusion and unsteadiness, diarrhoea, vomiting and agitation.

If left untreated, heatstroke can be fatal for dogs.

Based on their findings, recently published in the Open Veterinary Journal, the researchers suggest annual campaigns to raise awareness of the risk of dogs becoming ill in hot cars, which usually begin in May, need to start earlier in the year.

Dr Anne Carter, a researcher at Nottingham Trent University’s School of Animal, Rural and Environmental Sciences, and first author on the study, said: “Our work shows an even bigger risk to leaving dogs in parked vehicles than previously thought.”

She added: “People assume the risk is only midday during the summer, when in fact cars can reach potentially dangerous temperatures all year round, with late afternoon the hottest time period.”