A U.S. judge has entered a not guilty plea on behalf of Colorado cinema massacre suspect James Holmes after he failed to respond to charges before a court-appointed deadline.

The 25 year old stands accused of killing 12 people and injuring 70 others after opening fire at a midnight screening of The Dark Knight Rises in Aurora last July (12).

Holmes is charged with a total of 166 counts, including murder and assault, but on Tuesday his lawyers declared their client wasn't ready to address the allegations, prompting Judge William Sylvester to enter the default plea, which can be changed to not guilty by reason of insanity at a later date.

The former graduate student's next hearing has been set for 1 April (13), when the local District Attorney will announce whether prosecutors have decided to seek the death penalty.

Holmes' trial is due to begin on 5 August (13).