Lawyers representing the accused gunman behind last summer's (12) Colorado cinema massacre have filed their intent to enter a plea of not guilty by reason of insanity on behalf of their client.

James Holmes faces charges of killing 12 people and wounding dozens more after allegedly opening fire in a packed Aurora theatre as film fans were waiting for the start of the Batman sequel The Dark Knight Rises.

Prosecutors, who are seeking the death penalty in the case, claim the 25 year old had been planning the massacre for months.

Holmes' attorneys have previously offered to file a guilty plea on their client's behalf if prosecutors agreed to spare his life and push for life imprisonment instead, but Arapahoe County lawmakers declined the offer.

Previously, a judge entered a standard plea of not guilty for Holmes.

His trial is scheduled to begin in February (14).