OTTAWA - Robert Pickton was the "head honcho" in the murders of six Vancouver prostitutes and any mistakes at his trial should not undo his lawful conviction, the Supreme Court of Canada heard Thursday.

British Columbia prosecutor John Gordon told the high court no errors were made by the judge at Pickton's 2007 trial - and even if there were errors, they were so minor they do not warrant a new trial.

Gordon was rebutting earlier arguments by Pickton's lawyer, Gil McKinnon, that the former pig farmer did not receive a fair trial when he was convicted of six second-degree murder charges in the deaths of sex-trade workers in Vancouver's squalid Downtown Eastside.

The Crown contends its case against Pickton was so overwhelming that he couldn't possibly avoid conviction at another trial, so justice would not be served by ordering a new one.

Pickton's appeal hinges on instructions the trial judge gave to jurors after they returned with a question on the sixth day of deliberations. They wanted to know whether they could still convict Pickton if he was aided by an accomplice.

The Supreme Court reserved judgment Thursday but not before a full panel of justices grilled prosecution and defence lawyers during the two-hour hearing.

Debate centred on whether the initial instruction by the trial judge was adequate, and whether his subsequent answer to the jury muddied the waters further, possibly undermining Pickton's constitutional right to fully answer the charges against him.

"There was a failure, a procedural error, in failing to qualify the question," said McKinnon.

Justice Louise Charron raised the issue of whether an ideal set of instructions would have made a difference to the trial's outcome.

"How could a full aiding-and-abetting instruction from the word get-go possibly have benefited Mr. Pickton?" she asked.

"I don't say it could have," replied McKinnon.

"So we would send him back for a new trial for something that could not possibility benefit him?" Charron answered.

Justice Louis Lebel noted that the "root cause" of the jury's uncertainty was the failure of the trial judge to give proper instructions in his original charge.

Gordon said the trial prosecutors made a mistake by relying on the too-narrow theory that Pickton acted alone. Yet evidence emerged at trial of the involvement of others in the killings, and none of it diminished Pickton's guilt.

"He was, as he admitted, the head honcho," Gordon argued.

The jury that convicted him realized that Pickton was "the one constant at each and every stage of the scheme, at each and every instance of its execution," Gordon told the justices.

Pickton is serving a life sentence with no possibility of parole for 25 years.

Pickton was convicted after an 11-month trial. He was originally charged with 26 counts of first-degree murder, but the judge severed the other 20 cases from his trial.

British Columbia's attorney general has said if Pickton's appeal is successful, it would retry him on all 26 counts.