VANCOUVER -- A judge has ruled British Columbia Crown lawyers can proceed with legal arguments aimed at indefinitely locking up a mentally ill man who killed his three children.

But their bid to have Allan Schoenborn designated a "high-risk accused" won't be heard until his lawyers mount a constitutional fight against the law enacted by the former Conservative government in 2014.

B.C. Supreme Court Justice Martha Devlin ruled that public protection is the top consideration in allowing prosecutors to argue that changes to the Criminal Code should apply to Schoenborn.

Early last month, defence lawyers told the court that Bill C-14 should not be applied in Schoenborn's case because the law was passed years after the 2008 killings.

But in a ruling issued Wednesday, Devlin rejected the argument.

"Public protection is the paramount consideration ... the public must be shielded against those (not criminally responsible) accused who are considered dangerous and pose an unacceptable risk to society," Devlin wrote in her decision.

The B.C. Review Board decided last spring that the hospital director has discretion to grant him escorted outings into the community. The Crown declined to appeal that decision but instead mounted its objection by pursuing the "high-risk" designation.

Former prime minister Stephen Harper used Schoenborn's case to promote Bill C-14 and personally announced the new law at a news conference with the family of Schoenborn's victims in February 2013.

The law includes provisions stopping almost all absences from a psychiatric hospital and has the potential to extend annual review hearings to once every three years.

Schoenborn was found not criminally responsible on account of mental disorder after stabbing his 10-year-old daughter and smothering his eight and five-year-old sons in their Merritt, B.C., home.

Rishi Gill, one of two lawyers who represent Schoenborn, said they will appear next in court for five days in April to argue that Bill C-14 is unconstitutional.

He said the new law violates Schoenborn's right to be treated as a mentally disordered individual.

"We say that is not consistent with his rights that are enshrined in the charter," he said in an interview after the decision was released. "We say that he is in fact being treated like a criminal, which is how the former prime minister described him."

Both the defence and several of the country's top mental health experts objected when Harper called Schoenborn a "criminal" in a radio ad during the federal election campaign.

A spokesman for family members of the three children killed said they are "very happy" with the judge's decision.

"The result is a resounding 'Yes' for victims all across Canada," Dave Teixeira told reporters outside a court in New Westminster, B.C.