DENVER (Reuters) - Accused movie theater gunman James Holmes has been ordered to stand trial on charges he shot 12 people to death and wounded dozens more at a midnight screening of a Batman film last summer, but he is unlikely to enter a plea at a Friday court appearance.

His lawyers informed Colorado state court Judge William Sylvester on Thursday that they were not ready to proceed to an arraignment of their client on the scores of criminal counts leveled against the 25-year-old former graduate student.

Sylvester ruled a short time later that evidence presented by prosecutors during a three-day preliminary hearing this week had established probable cause to believe that Holmes committed the crimes, and ordered him bound over for trial on all counts.

The judge said Holmes, described by his own lawyers as suffering from an unspecified mental illness, should remain held without bail and indicated he would grant a postponement for Holmes' arraignment.

The ruling followed three days of wrenching testimony about the shooting, its bloody aftermath and the elaborate preparations that Holmes is accused of making for the attack.

Some legal experts say the proceedings left Holmes' lawyers little choice but to mount an insanity defense for their client.

"The defense team has nowhere else to go given the obvious premeditation and overwhelming evidence against Holmes," said Craig Silverman, a former Denver prosecutor now in private practice as a trial attorney.

The former neuroscience doctoral student is charged with multiple counts of first-degree murder and attempted murder stemming from the July 20 rampage at the opening of "The Dark Knight Rises" in the Denver suburb of Aurora.

In addition to the 12 people who died, 58 others were wounded by gunfire and a dozen more suffered other injuries.

The tragedy stands as one of the deadliest mass shootings in U.S. history and one that ranked briefly as the most lethal in 2012 - until 20 children and six adults were killed last month at a Connecticut elementary school.

QUESTIONS OF MENTAL STATE LOOM LARGE

In a legal brief opposing a media request to allow cameras in the courtroom, Holmes' team said they were not prepared to enter a plea in the sensational case.

Sylvester strongly suggested in his ruling that he was apt to grant a postponement, if requested, on Friday.

"In a case like this, a judge will give the defense every opportunity to take the time they need so there's not the potential for a reversal on any legal issue," former Colorado district attorney Bob Grant told Reuters.

Defense lawyers could also seek a hearing to determine if their client is legally competent for trial or incapable of understanding the courtroom proceedings and assisting in his own defense.

Even if Holmes is fit for trial, his lawyers could enter an insanity plea arguing he should not be held legally culpable for his actions due to his mental state at the time of the attack.

Under Colorado law, Silverman said, prosecutors would bear the burden of proving Holmes was not insane. However, juries have typically shown reluctance to accept an insanity defense, he said.

Silverman said prosecutors presented compelling evidence that Holmes knew right from wrong when they showed his online dating profile with the headline: "Will you visit me in prison?"

Holmes is accused of entering Theater 9 of the Century 16 multiplex with a ticket he bought 12 days in advance, then leaving through a rear exit minutes into the movie and re-entering moments later wearing body armor and a gas mask.

Armed with a shotgun, pistol and semi-automatic rifle, authorities say, Holmes lobbed a tear gas canister into the auditorium and sprayed moviegoers with bullets until one of his guns jammed, then surrendered to police without a struggle in the parking lot behind the theater.

Police testified that Holmes began assembling his collection of guns and ammunition two months before the shooting, scouted out the multiplex weeks ahead of time, and took photos of his arsenal and of himself posed with weapons and body armor.

Holmes had booby-trapped his apartment near the theater with explosives, which police said he intended as a diversion to draw authorities away from the movie house while he was carrying out his assault. The bombs were later defused safely.