By an 8-1 margin, the justices rejected Linda Dale Gibbons's argument that she should never have been charged under the Criminal Code for picketing too close to a Toronto abortion clinic.
The high court did not wade into the emotionally and politically charged abortion debate, but instead confined itself to a single, technical, legal point: whether Gibbons should be dealt with under criminal or civil law.
Gibbons maintains she should have been dealt with under civil law for disobeying a court order, not charged criminally.
She was charged under section 127 of the Criminal Code. She argued that she should have been entitled to an exemption under that section that would have taken her case out of the criminal courts.
The justices disagreed.
The ruling means she will go to trial if the prosecution decides to pursue the case.
Gibbons' lawyer Daniel Cantor said he doesn't yet know whether the Crown plans to proceed with the case against his client.
"Ms. Gibbons still stands charged with disobeying an 18-year-old temporary court order," he said.
"This is an abuse of process, and she will continue to pursue that argument in the courts."
The 63-year-old has been picketing abortion clinics for almost two decades and has spent a total of nine years behind bars intermittently.
She was arrested again last December for picketing too close to a clinic.
That arrest came just two days after her lawyer argued her case before the Supreme Court.
The case she lost on Friday dates back to 1994, when an Ontario court ordered her and others to stay more than 20 metres away from Toronto abortion clinics.
The injunction came six years after the Supreme Court struck down Canada's abortion law and two years after the Morgentaler abortion clinic in Toronto had been fire bombed.
In October 2008, Gibbons was charged with breaching the injunction for picketing and displaying a sign within the buffer zone.