LOS ANGELES (AP) -- The long-awaited trial of the doctor charged in Michael Jackson's drug death was delayed Monday for four months, with a judge saying defense lawyers needed additional preparation time to effectively represent their client.

Superior Court Judge Michael Pastor said he was more concerned with justice for Dr. Conrad Murray than with the speed of the proceedings.

Murray, who had insisted on a speedy trial, waived that right and agreed to start jury selection anew on Sept. 8. Lawyers estimated opening statements would begin Sept. 20.

Attorneys for Murray filed a motion Sunday complaining about the last-minute addition of expert witnesses to the prosecution case and saying they needed at least two weeks to find experts of their own and have them prepare reports.

They also are trying to get raw footage from Jackson's posthumous concert movie, "This Is It." Prosecutors plan to use clips to show Jackson was in good health just before he died.

Pastor said he did not think two weeks was sufficient for the defense to accomplish its goals.

Prospective jurors who filled out questionnaires and were to return to court Tuesday will be thanked and told their services are no longer needed, the judge said.

"The court is very mindful of judicial efficiency and the expense that has occurred in this case," Pastor said. "But first and foremost is justice ... The continuance in this case is absolutely essential."

Pastor said he didn't like the idea of picking a new jury but said it was "preferable to having this continuing drama."

Deputy District Attorney David Walgren said he had no objection to a delay. He had been urging a later start date all along.

Murray has pleaded not guilty to involuntary manslaughter. He is accused of negligence in administering the hospital anesthetic propofol to Jackson in his home. The superstar singer died June 25, 2009, of an overdose of propofol coupled with other sedatives.

The Murray defense has posed the theory that Jackson, who was desperate for sleep, swallowed more propofol than the doctor gave him. The drug normally is administered intravenously.

The prosecution presented new reports last week from two experts who say it's impossible to overdose by taking the drug orally because it would not be absorbed into the intestines.

Murray, who had been adamant about wanting a trial within 60 days of his preliminary hearing, was asked by the judge if he was giving up that right to allow his lawyers to effectively represent him.

"Yes," Murray said. "I believe it's in the best interest of all parties."

"But do you feel it is in your best interest?" asked the judge.

"Yes," said the defendant.

Pastor said he planned to keep the case "on a short leash" and set a progress hearing on June 3.

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Associated Press Entertainment Writer Anthony McCartney contributed to this story.