A Quebec provincial police officer has been found guilty of dangerous driving in the death of a five-year-old boy on Montreal's South Shore.

Patrick Ouellet was in an unmarked police cruiser, tailing a suspect at more than 100 km/h in a 50 km/h zone, when he struck and killed Nicholas Thorne-Belance four years ago in Saint-Hubert.

In a ruling issued Thursday in Longueuil, Quebec court Judge Éric Simard said there was nothing to justify the speed at which Ouellet was driving and that there were "inherent" risks in such behaviour.

"His failure to take steps to avoid such risks constitutes a marked departure from the standard of care that a reasonable person in the same situation would follow," Simard wrote in his decision.

Part of surveillance operation

At the time of accident, in February 2014, Ouellet was following a politician as part of a surveillance operation targeting provincial corruption.

He reached a speed of 134 km/h on Gaétan-Boucher Boulevard in the moments before the accident — and 108 km/h at the moment of impact.

At the intersection of Gaétan-Boucher and Davis, he struck a vehicle carrying two children in the backseat. Nicholas was critically injured and died in hospital five days later.

Nicholas Thorne-Belance died in hospital five days after a speeding unmarked police cruiser crashed into his father's vehicle while he was a backseat passenger. (CBC)

During the trial, Ouellet testified that the crash was unavoidable. However, the Crown argued the officer's driving behaviour that day was "objectively dangerous."

Family 'very satisfied' with decision

Stephanie Thorne, the boy's mother, broke down in tears after hearing that Ouellet had been found guilty. The family did not offer comment outside the courtroom.

The boy’s family is huddling outside the courtroom, hugging and crying. There seems to be a sense of relief. —@TurnbullJay

Ouellet, with his shoulders slumped, clutched his girlfriend's hand throughout the judge's decision. After the ruling, when the judge ordered a short recess, she stayed inside the courtroom and wept.

Nadine Touma, Ouellet's lawyer, declined to comment but said the defence would appeal the judge's decision.

The Crown initially decided to not lay charges, but then reversed its decision in 2015 after Justice Minister Stéphanie Vallée ordered an independent review of the case amid intense pressure.

After the boy's death, a sign reading 'Speeding is criminal!' was installed at the intersection where the collision occurred. (Sarah Leavitt/CBC)

Crown prosecutor Geneviève Langlois said the family was "very satisfied" with Thursday's ruling.

"They had the impression that justice was rendered today, and for them it's a big step regarding all the steps in the four years between the event and today," she said.

Convictions for dangerous driving causing death carry a maximum penalty of 14 years in prison.

Sentencing is set for Oct. 22.