The civil case, which began almost 13 years ago, pitted a former resident of Whitby, Ont., against the Durham Region Police Service.
Ontario Superior Court justice Douglas Gray found that an officer with the force, James Liepsig, promised Margaret Stack her identity would not be disclosed if she came to a police station and provided information about suspected criminal activity.
"He did not qualify that promise in any way. Thus, both expressly and by implication Ms. Stack became entitled to informer privilege, that is, she was entitled to have her anonymity preserved," Gray wrote in his decision.
But Stack's identity was not kept confidential, Gray found, and was in fact revealed to the family that was the subject of her tip, triggering weeks of harassment.
"The effect on Ms. Stack has been severe. Her life has been irretrievably altered," Gray wrote. "On a balance of probabilities, I am satisfied ... that it is unlikely that she will ever be the same as she was before these incidents."
Stack's lawyer said Gray's decision was thought to be the first civil case in which the court condemned police for not protecting an informant and found a force liable for the harm that ensued.
"The case is important because it does show that the courts will protect individuals that are given promises by the police in the course of doing their public duties such as providing information," said Margaret Hoy.
"Central to the justice system is people assisting police by providing information, because without them we would not have information to investigate and prosecute crimes."
A spokesman for Durham police said a notice of appeal has been filed as a result of the decision.
The incidents that sparked the case took place in 2002.
Stack was told by a neighbour that two brothers who lived across the street had allegedly broken into a home, stolen guns that were in the residence and taken them to school, where they threatened students, Gray's decision said.
One of those brothers, neither of whom can be named, used to babysit for Stack's own children.
Worried about the safety of her family, Stack wanted to tell police but was reluctant to have the information traced back to her. A friend told her he knew an officer he could tip off.
Stack then got a phone message from Liepsig asking to speak with her — a development which left her "shocked," the court decision noted. When she asked her friend why he had disclosed her identity, he said Liepsig had become "heavy" with him.
"Ms. Stack testified that officer Liepsig said that if she came to the police station and talked to him, he would keep her identity secret. He said he would protect her and she would be totally anonymous," Gray wrote.
Liepsig testified there was no discussion about informer status for Stack or confidentiality. He also testified she asked for no assurances in exchange for giving police her statement.
One of the boys was arrested and Stack testified she noticed increasingly hostile behaviour from his family.
At one point, Stack testified the boy's father drove his truck at her. She told court the truck would have hit her if she hadn't rushed out of the way.
Stack then left a phone message for Liepsig saying she feared her identity had been revealed. She testified the call was never returned.
Her husband also had a phone conversation with the neighbour driving the truck in which it became apparent Stack's identity as an informant had been revealed.
After that, court heard the family across the street "started to watch them constantly," standing on the front step of their house and driveway and glaring at them — Stack and her family started entering their home through the back.
Stack's family ended up selling that home and moving away, and Stack was diagnosed with post traumatic stress disorder.
"Had Ms. Stack's identity not been disclosed, she and her family, in all likelihood, would still be living in their dream home on their quiet street," Gray wrote.