The Supreme Court of Canada has rejected serial killer Robert Pickton's bid for a new trial.

The top court was unanimous in ruling that Pickton's right to a fair trial was not affected by the trial judge's final instructions to the jury, although they split 6-3 on the reasons.

Pickton was found guilty of six counts of second-degree murder in December of 2007 and is charged in 20 other deaths.

His trial took almost a year, but the appeal centres around a question the jury asked the trial judge on their sixth day of deliberations.

The panel wanted to know whether they could find Pickton guilty if they inferred he acted indirectly.

In his answer, the trial judge said if they found Pickton "was otherwise an active participant" in the killings, they could find him guilty.

Pickton's defence had argued that the judge's answer gave the jury an avenue to convict their client without giving them a chance to defend him properly, as the Crown's case rested on Pickton being the only one responsible for the crimes.

In writing for the majority, Justice Louise Charron said that didn't happen:

"The fallacy of Mr. Pickton's argument lies in the fact that the defence theory itself put the participation of others at issue. Throughout the trial, the defence by its approach urged the jury to consider that others may have actually killed the victims.'

"An inevitable consequence of going down that road is that the jury would have to be instructed on how this could, if at all, impact on Mr. Pickton's own criminal liability."

In fact, Charron wrote, the judge would have made a mistake if he had not clarified his instructions, because his initial charge was confusing.

"The adequacy of the jury instructions must be assessed in the context of the evidence and the trial as a whole. There is nothing wrong, particularly in complex or lengthy trials, with the trial judge and counsel's narrowing the issues for the jury by focusing on what is actually and realistically at issue in the case, provided that, at the end of the day, the jury is given the necessary instructions to arrive at a just and proper verdict.

"Realistically, this case was never about whether Mr. Pickton had a minor role in the murder of the victims. It was about whether or not he had actually killed them."

Pickton was convicted of second-degree murder for the deaths of Mona Wilson, Sereena Abotsway, Brenda Wolfe, Marnie Frey, Andrea Joesbury and Georgina Papin.

He was sentenced to life in prison with no possibility of parole for 25 years.

The former pig farmer had been charged with 26 counts of first-degree murder, but the trial judge split the charges into two groups and a trial on the remaining 20 charges was put on hold pending the top court's decision.

The Crown in B.C. had won an appeal in a lower court that would have allowed them to retry Pickton on all 26 counts of first-degree murder had the Supreme Court overturned the conviction.

It is now expected that the other 20 charges will be stayed.

Pickton's trial followed one of the most massive police investigations in Canadian history, involving thousands of pieces of evidence.

The severed heads, hands and feet of three of the women were found in buckets on his Port Coquitlam, B.C. pig farm, while the bones and DNA of the others were scattered across the property.

The jury heard from Pickton's associates that he bragged about picking up prostitutes and luring them back to his farm to kill them and also heard from Pickton himself, in the form of a lengthy interview with police and with an undercover officer in his jail cell.

In the conversations, he appeared to brag about being the head honcho, and also that he had killed 49 women and wanted to make it one more for an even fifty. He also said he had gotten sloppy towards the end.

In their decision, the Supreme Court also addressed whether the trial judge should have more fully instructed the jury on the issue of aiding and abetting.

Writing for the minority, Justice Louis LeBel said he should have.

"The re-charge whereby the trial judge instructed the jury that they could convict Mr. Pickton if they found he was the actual shooter or `was otherwise an active participant' in the killings clearly opened up party liability as an alternate route to conviction," he wrote.

"That having been done, it was an error for the trial judge not to have left a full aiding and abetting instruction with the jury in order to set out the alternate route properly by which the jury could convict Mr. Pickton for the six murders with which he was charged."

In the end, LeBel said: "Certainly, this was a long and difficult trial -- but it was also a fair one.'

"Despite the errors set out above, there was no miscarriage of justice occasioned by the trial proceedings. Mr. Pickton was entitled to the same measure of justice as any other person in this country. He received it. He is not entitled to more."