Image copyright Facebook Image caption Justin Ross Harris and his son, Cooper

A US father who deliberately left his toddler son to die in a hot car has been sentenced to life in prison without possibility of parole.

Justin Ross Harris, 36, was found guilty last month of the 22-month-old boy's death in June 2014 in a car park outside his office in Atlanta, Georgia.

He argued the death was an accident and he forgot to drop the boy at nursery.

But the five-week trial heard he was sending lewd text messages to underage girls as the boy was dying.

Image copyright Facebook Image caption Cooper died of hyperthermia as his body overheated amid outside temperatures of almost 90F (32C)

Cobb County Superior Court Judge Mary Staley Clark said it was a "horrendous, horrific" crime, as Harris had "callously walked away and left that child in a hot car to swelter and die".

She added an extra 32 years - the maximum penalty allowed - in line with prosecutors' recommendation.

Wearing an orange jail-issue jumpsuit, Harris stood expressionless with feet shackled as he learned his fate on Monday.

The former Home Depot website developer was convicted in November on eight counts, including malice murder as well as charges of child cruelty and sex crimes related to the text messages he sent to a teenage girl.

Prosecutors had argued Harris plotted to kill his son because he wanted to leave his family in order to continue affairs with other women.

Image copyright AP Image caption Harris thought he had dropped his son at nursery, the trial heard

Harris' lawyers said that though he was responsible for the boy's death, he loved his son and the incident was the result of tragic forgetfulness.

The children left behind in hot cars

Cooper died of hyperthermia as his body overheated while sitting in the car for about seven hours amid outside temperatures of almost 90F (32C).

Cobb County Police Detective Phil Stoddard had testified in court that Harris was sitting in his office, exchanging nude photos with several women, including a teenage minor, on the day his son died.

Image copyright AP Image caption Leanna Taylor testified in defence of her ex-husband

But reports early in the investigation that Harris had researched online how long and what temperature it would take to die in a hot car were found not to be true.

Instead the court heard that he read a Reddit post on this subject that was shared with him by someone else, but did not search for it specifically.

It was also denied that he researched websites advocating a child-free lifestyle. He was sent a link to one such site by a co-worker, the court was told, but responded with the words: "Grossness, no thanks."

Friends and family testified on Harris' behalf, including his ex-wife, Leanna Taylor, who divorced him after the arrest.

She said while he "destroyed" her life, he was a loving father who would not intentionally harm their son.

During the trial, Harris cried when photos of his son were shown to the jury.