MILTON, Ont. - Premier Dalton McGuinty says his government is taking steps to stop secret background checks on jurors that critics say could jeopardize court cases across Ontario.

Judges in Barrie and Windsor have declared mistrials because the Crown asked police to screen potential jurors, and lawyers are warning that it could unleash a flood of defence challenges.

McGuinty says all Crown offices have been told that they don't have the authority to do those kinds of secret screenings and to follow the rules from now on.

The mistrial in Windsor comes after two jury panels in Barrie were sent home after the defence argued that the Crown had tainted them by enlisting police to conduct a wide-ranging vetting of their backgrounds.

The NDP says a large number of convictions are now in doubt and Attorney General Chris Bentley has to come clean about how long this practice has been going on.

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.