A US judge has accepted of plea of not guilty by reason of insanity from the man accused of last summer’s Colorado cinema shootings.

The decision sets the stage for a lengthy mental evaluation of James Holmes, who is accused of killing 12 people and injuring 70 in a packed Denver-area screening of a Batman movie last July.

The evaluation could take months. Holmes is charged with multiple counts of murder and attempted murder, and prosecutors are seeking the death penalty.

Holmes’s lawyers have repeatedly said he is mentally ill, but they delayed the insanity plea while arguing that state laws were unconstitutional. They said the laws could hobble the defence if Holmes’s case should ever reach the phase where the jury decides if he should be executed.

The judge rejected that argument last week.

The July 20 massacre was one of several that jolted the debate over gun violence in the US, and it prompted Colorado to adopt significant state-wide gun controls this year.

Hundreds of people were watching a midnight showing of The Dark Knight Rises at the Aurora cinema when the shooting occurred.

The dead included a navy veteran who threw himself in front of friends to shield them, an aspiring sports journalist who had survived a mall shooting two months earlier, and a six-year-old girl.

Prosecutors say Holmes spent months buying weapons, ammunition and materials for explosives and scouted the cinema in advance. He donned police-style body armour, tossed a gas canister into the seats and opened fire, they say.

The insanity plea is widely seen as Holmes’s best chance of avoiding execution, and possibly his only chance, given the weight of the evidence against him.

Colorado law defines insanity as the inability to distinguish right from wrong caused by a diseased or defective mind.

If jurors find Holmes not guilty by reason of insanity, he would be committed indefinitely to the state mental hospital. He could eventually be released if doctors find his sanity has been restored, but that is considered unlikely.

If jurors convict him, the next step is the penalty phase, during which both sides call witnesses to testify about factors that could affect why Holmes should or should not be executed.

The jury would then decide whether Holmes should be executed or sentenced to life in prison without possibility of parole.