Lawyers for Robert Pickton, seen in this February 2007 illustration, say the trial judge's instructions confused jurors. (Jane Wolsak/Canadian Press)

The Supreme Court has reserved judgment on whether convicted serial killer Robert Pickton should get a new trial.

Lawyers representing Pickton appeared before the Supreme Court of Canada Thursday in a bid to win a new trial for their client, arguing that the trial judge made an error in his instructions to the jury.

Pickton, a former B.C. pig farmer, was convicted in the murders of six prostitutes in 2007. He was sentenced to life in prison with no parole for at least 25 years.

Canada's highest court is expected to rule on the appeal in the coming months.

Pickton's lawyer, Gil McKinnon, argued that the judge's instructions confused the jury and compromised Pickton's right to a fair trial.

At one point during original trial deliberations, the jury wrote a question asking B.C. Supreme Court Justice James Williams if they could convict Pickton if they believed he hadn't acted alone.

Williams had crafted his initial charge to the jury after consulting with the prosecution and defence, telling the jury it had to render a guilty verdict if it found Pickton was the actual shooter.

But Williams revised his instructions to the jurors in response to the question, telling the jury they could still convict Pickton if they found he was the shooter or was an "active participant" in the killings.

Jean Cumming, editor-in-chief of Lexpert, a legal magazine, said there is "considerable disagreement" between the two sides about the judge's instructions.

"We don't know whether the jury was properly instructed," Cumming said, noting that questions around the judge's instructions are the "core" of the hearing.

The defence argued that the Crown, at trial, rejected suggestions that Pickton might have had an accomplice.

"Put simply, the jury's critical question was incorrectly answered, the 'goal posts' were changed by the amendment at a very late and impossible stage of the trial, and the Crown gained a significant, unjustified advantage," Pickton's lawyers say, adding that his conviction "might even fall outside the ambit of criminal liability."

Crown argues against new trial

Police took this aerial photo of the Pickton farm during their 2002 investigation. (Canadian Press)In their submission, Crown lawyers argue that the case against Pickton was so overwhelmingly strong that he would be reconvicted if a new trial were ordered. Therefore, they argue, his appeal should be rejected.

"He is a self-confessed killer. He admitted he was caught because he became sloppy at the end," the Crown says in a brief.

"Ordering a new trial in this case, where conviction is inevitable, would serve only to 'detract from society's perception of trial fairness and the proper administration of justice.' "

Pickton was convicted in the deaths of six women from Vancouver's Downtown Eastside. He was originally charged with 26 counts of first-degree murder but the judge severed the other 20 cases from the trial.

The Crown has said if a new trial is ordered, Pickton will be retried on all 26 counts.

Pickton won the right to appeal to Canada's top court after the B.C. Court of Appeal issued a split 2-1 ruling in June 2009 upholding his conviction, saying the mistakes made by the judge were not serious enough to warrant a new trial.

With files from The Canadian Press