A woman was arrested after leaving her year-old grandson in what police said was a "baking hot car" while she went shopping.
The incident happened in Whitley Bay on 23 July, when the baby was left in a car in temperatures approaching 30C.
Passers-by alerted police to the child, who was alone in the car, parked at the Preston North Road branch of Morrisons.
The child was not hurt, but officers cautioned the grandmother, who has not been named.
Northumbria Police has now issued a warning to parents and pet owners to use "common sense" during the current spell of hot weather.
Dangerous consequences
A force spokesman said that conditions within vehicles can see temperatures rise an additional 10C degrees.
Insp Tony Blacklock added: "We were called to Morrisons car park in Preston North Road on Sunday morning, after a passer-by noticed a baby had been left alone in a car.
"On arrival we found a one-year-old boy in the car and a few minutes later his grandmother returned from doing her shopping.
"Thankfully the boy had not suffered any injuries and the grandmother was arrested and cautioned for neglect, but the consequences could have been much worse.
"Motorists know themselves how quickly the temperature can build up inside a car.
"Children, particularly very young ones, should never be left alone in cars whatever the weather, but to do so in the current hot spell is sheer stupidity."