A jury in the US state of Colorado has spared gunman James Holmes the death penalty for killing 12 people at a screening of a Batman film in 2012.

He will serve life in jail without the possibility of parole.

The defence team had argued that the former neuroscience graduate student, now 27, was insane at the time.

The jury agreed with prosecutors that Holmes, though mentally ill, was responsible for his actions. But it was not unanimous on the death penalty.

That lack of agreement meant the jury accepted he would receive an automatic life sentence without parole.

One juror later told NBC News that two members of the jury were "on the fence" about the death penalty but that another was adamantly opposed on the grounds of mental illness.

"We ended our deliberations when one absolutely would not move," the juror said.

The decision of the jury - a panel of nine women and three men - was revealed by Judge Carlos Samour in a courtroom in the city of Centennial on Friday.