The Supreme Court of Canada has ruled that police officers under investigation by a civilian watchdog are not allowed to have a lawyer help them prepare their notes.

The high court made the ruling by a 6-3 margin in a case that pitted officers against the families of two men shot dead by Ontario's provincial police in separate incidents in 2009.

The ruling offers clarity to the regulations that govern the province's Special Investigations Unit, which investigates violent incidents involving police officers.

Justice Michael Moldaver, writing for the majority, said public trust in the police is of "paramount" concern.

"This concern requires that officers prepare their notes without the assistance of counsel," the ruling said.

"Consultations with counsel during the note-making stage are antithetical to the very purpose of the legislative scheme — and for that reason, they must be rejected."

The families argued before the Supreme Court that having a lawyer approve the notes that end up in police memo books is unacceptable.

They also said that allowing a single lawyer to act for several officers involved in an incident undermines rules against collusion.

Police argued they have the right to talk to a lawyer of their choosing before finalizing their notes.