Elections Ontario says personal information about as many as 2.4 million voters in the province may have been compromised.
Chief Electoral Officer Greg Essensa says the privacy breach involves voters in up to 25 ridings.
He says experts are trying to determine which ridings were affected among 49 possible electoral districts.
Essensa says data was on two memory sticks and the information was not encrypted or password protected.
Names, birth dates, addresses and gender of voters were among the data that was collected in last fall’s election.
Ontario’s Information and Privacy Commissioner says the breach involves an “unprecedented” number of people.
The data may also include information on whether or not these same individuals voted, but not how they voted.
Ontario Provincial Police are investigating and the three major political party leaders have been notified.