A 5-year-old Cyprus boy was found dead in the family car, forgotten there for nearly eight hours by his doctor father while he carried out emergency surgery, police said on Wednesday.

An autopsy indicated that the boy could have died from a heart attack and not from heatstroke or asphyxiation as first feared.

"The child had quite serious heart problems that were discovered during the autopsy. For a final conclusion we must await the results of tissue tests," state pathologist Eleni Antoniou told reporters.

She said the child had been dead for six hours before he was found, ruling out the June heat as a factor and suggesting an unknown heart defect was the cause of death.

"Small children can withstand high temperatures for many hours. The possibility of death by heatstroke or asphyxiation is excluded here."

Police said they found the boy dead at 5:00pm local time on Tuesday afternoon in the car parked outside a private clinic in the southern coastal town of Larnaca.

According to the Cyprus News Agency (CNA) the police found the car locked, the boy was in the back seat and there was an empty water bottle next to him.

The father had planned to take the boy to daycare but on the way he received a call asking him to rush to the hospital to conduct emergency surgery.

The alarm was only raised when the mother went to pick the boy up from daycare and was told he never arrived.

It was only then the 40-year-old father realized that he had not taken his son out of the car.

The boy's funeral will take place Wednesday afternoon in Larnaca.

The family have two other children, age 7 and 9.