Using the terms "shocked" and "disgusted", Marc Parent told a news conference the force had been badly shaken by the arrest of former organized crime investigator Benoit Roberge last Saturday.
When he was stopped he was allegedly in the company of an individual linked to the Hells Angels crime gang.
Montreal police brass had no idea Quebec provincial police were investigating Roberge, who retired this past August.
"There are colleagues who have not slept since his arrest on Saturday, knowing that a credible police officer with whom they had shared information had betrayed their trust," Parent said.
"How to live with that when you realize you've never seen any signs? There's nothing to tell you that this person could have gone to the dark side?"
Calling it an "exceptional and isolated case," Parent said Roberge's former colleagues have been left reeling. The police chief said he has met with 200 investigators, some of whom worked closely with him.
"Everyone feels betrayed," said Parent. "The only consolation is that he was caught and was brought to justice, but it's a small consolation in the face of the damage he's done to police organizations and to the administration of justice."
Roberge appeared in court on Monday on four charges: one of obstructing justice, one of breach of trust and two related to gangsterism. His bail hearing is scheduled for Thursday.
Roberge, 50, was a sergeant-detective who worked for 11 years in specialized squads that investigated organized crime, He teamed with RCMP and provincial police officers, among other forces, and his last five years were spent investigating criminal bikers.
He also testified at biker trials before he retired. Roberge has been suspended from a job working in intelligence for the province's tax collection agency.
Further complicating matters is that Roberge's spouse is a Crown prosecutor specializing in organized crime. The Quebec prosecutor's office has announced an increase in security measures to protect its files. A prosecutor from out of town has been brought in to handle the Roberge case.
In a statement issued late Wednesday, the prosecutor's office said Roberge's spouse has been temporarily stripped of her responsibilities and that she no longer has access to Crown files.
The statement said she has co-operated with provincial police investigators and that there is nothing to believe she was involved in any of the alleged activities that led to the charges against Roberge.
The police chief added that in the aftermath of another sensitive information scandal in 2012, the force instituted new security measures and sought to tightly control sensitive information with more cameras, random polygraph testing and tougher rules about who could have such information at their disposal.
The Montreal police detective implicated in that scandal killed himself in January 2012 while under investigation over whether he peddled a list of police informants to the Mafia. Ian Davidson took his own life after being outed as a mole in different newspaper reports.
This time, Parent said it may be time to discuss how to treat sensitive information on a national scale. He co-chairs a national police body that looks at intelligence matters.
"All major dossiers are now shared among police forces; organized crime really has no boundaries," Parent said.
"Investigations often go past our own borders and it's an opportunity to develop actions and guarantee better security in cases like the Montreal police is going through now."