Ontario's privacy czar launched a probe Wednesday into secret background checks on potential jurors, a practice which critics say could jeopardize court cases across the province.

The investigation will focus on whether privacy rights were violated when police used confidential databases to probe the background of would-be jurors for Crown prosecutors, said Information and Privacy Commissioner Ann Cavoukian.

The probe is also necessary because the practice seems to be more widespread than the government had first indicated, she added.

"Initially, the attorney general advised that this practice was restricted to one courthouse in Simcoe County," Cavoukian said in a release.

"Since then, we have learned that the practice extends to at least one other courthouse, and possibly others. As a result, we will be conducting our own investigation into this matter."

The review will try to gauge how far the practice has spread, as well as examine what kind of personal information was collected, why it was gathered, who saw it, and whether it was adequately protected.

The controversy began last week in Barrie, where two jury panels were sent home before selection started after the defence argued that the Crown had tainted them by enlisting police to conduct background checks.

The practice also prompted counsel for Ibrahim Yumnu and two others who were convicted of first-degree murder and conspiracy to commit murder to ask Ontario's top court to re-open their appeal.

Then on Tuesday, a Windsor judge declared a mistrial in a first-degree murder trial — after two months of hearing evidence — because of jury vetting.

Ontario Superior Court Justice Bruce Thomas told jurors that he found the process in their case "to be offensive."

There's also a case in Thunder Bay where an "extensive check" was done, but wasn't used in selecting the jury and both parties agreed to proceed to trial, Attorney General Chris Bentley said Wednesday.

Under Canadian law, background checks on jurors are supposed to be conducted only for the most serious convictions. The Crown or defence counsel are only supposed to know the name, address and occupation of prospective jurors.

Critics warn the controversy could unleash a flood of defence challenges in Ontario, and a possible class-action lawsuit against police and the province.

"All these previous jury trials may be re-opened, just like it happened in the Dr. [Charles] Smith pediatric pathologist case, where they reopened all those trials where he gave testimony," said Greg Goulin, one of the defence lawyers in the Windsor mistrial.

"If it's found that it was done in other jury trials, it's possible that they could be reopened."

The province called a public inquiry into the disgraced pathologist's work, which criticized Smith last October for "irresponsible" testimony in a series of child death cases. Many of them were overturned in court.

In the Windsor case, city police provided the Crown with detailed information about potential jurors, including dropped charges and criminal convictions that should have been sealed, Goulin said. Two jurors were noted as "disliking" police.

The defence was provided with a jurors list that only included names, addresses and jobs, he said.

The information, obtained from a police database that is supposed to help them enforce the law, was used to "give the Crown an edge in picking a jury," Goulin said.

A large number of convictions are now in doubt and Bentley has to come clean about how long this practice has been going on, opposition parties said.

NDP justice critic Peter Kormos wants the matter to be investigated by the province's ombudsman, while interim Progressive Conservative leader Bob Runciman says it may require a full-blown inquiry.

"One fear is that it's a practice across the province," said Kormos.

"The attorney general was dancing a fine dance when he tried to leave the impression that it only happened in Barrie and that it had been curtailed some time ago — clearly, that's not the case."

Both Bentley and Premier Dalton McGuinty say the background checks are unacceptable and steps have been taken to stop the practice.

"They have been happening, obviously, in some cases, and all that we can do at this point in time is to make sure that they stop," McGuinty said.

The province's chief prosecutor has sent a directive to all 54 Crown jurisdictions telling them they must follow the rules and has phoned each one to find out if background checks were conducted, Bentley said.

If there are any such lists for ongoing cases, the Crowns have been instructed to disclose them to the defence and take "all necessary steps, including asking for new jury panels," he said.

"It was clear from the call-around that this was not a widespread practice," Bentley added.

However, he acknowledged that a virtually identical directive was sent out in March 2006.

Bentley said he welcomes the privacy commissioner's involvement and will co-operate fully with her investigation.

The Ontario Provincial Police also said it welcomed the probe.

"The OPP takes the privacy of information in our possession very seriously. We will work co-operatively with the information and privacy commissioner towards developing sound policies and practices to protect the confidentiality of personal information," said Commissioner Julian Fantino in a statement.