A Colorado judge presiding over the case of accused theatre gunman James Holmes entered a not guilty plea on his behalf on Tuesday to charges he went on a shooting spree in a Denver suburb nearly eight months ago and killed 12 moviegoers.

Arapahoe County District Judge William Sylvester made the move after declining a defence request for a continuance, paving the way for a trial to begin in August. Holmes' lawyers said they were not ready to enter a plea.

Holmes faces multiple counts of first-degree murder and attempted murder stemming from the rampage on July 20th that also wounded 58 people and was one of the deadliest mass shootings in U.S. history.

His lawyers are expected to mount an insanity defence for Holmes, who surrendered to officers outside the theatre in the Denver suburb of Aurora within minutes of the mass killing.

Prosecutors have depicted the former neuroscience graduate student at the University of Colorado at Denver as a young man whose once promising academic career was in tatters.

He failed graduate school oral board exams in June and one of his professors suggested he may not have been a good fit for his competitive doctoral program, prosecutors said.

Arapahoe County District Attorney George Brauchler said in court on Tuesday that his office would announce at some point during court hearings in the first week of April whether it will seek the death penalty for Holmes.

Sylvester said that, even though he was entering a simple not guilty plea on behalf of Holmes, the defence could later substitute a plea of not guilty by reason of insanity. Sylvester also scheduled a four-week trial for Holmes to begin on August 5th.

Presented by Adam Justice