TORONTO - The loss of medical records containing cancer screening tests for thousands of Ontarians, including names and health card numbers, is a very serious matter, Premier Dalton McGuinty said Wednesday.

Cancer Care Ontario reported Tuesday that health records for almost 6,500 patients were lost in the mail earlier this year, and the agency was still trying to find the test results for another 5,440 patients.

"We will do everything that we can to assist in whatever way possible to retrieve this information and ensure that there is ultimately no breach of confidentiality, and that this information does not get into the wrong hands," McGuinty told reporters.

Ontario's information and privacy commissioner, Anne Cavoukian, warned the data from the missing records could be used to help steal the identities of patients, calling them a "gold mine" for fraud artists.

Cavoukian said the health records should be delivered electronically, either on USB keys, CDs or other media that can be encrypted so the data is not easily accessible if stolen.

McGuinty dismissed suggestions Ontario was still using old fashioned paper records and mail to deliver them because the provincial agency, eHealth, spent about $1 billion to help create electronic health records but made very little progress in a decade.

"It does bolster the case, that we continue to make as a government, for us to move forward with electronic health records," said McGuinty. "One of the things that we'll be looking at Cancer Care Ontario for is improved and better ways to transmit that kind of confidential information."

Cancer Care issued an apology for the missing files Tuesday and said it used Canada Post's Xpresspost courier service to mail the records to doctors' offices. Xpresspost was supposed to get a signature confirmation for every package or return them undelivered.

The packages were part of the ColonCancerCheck program which aims to boost the number of Ontario residents aged 50 to 74 who are checked for colorectal cancer.

Cancer Care began looking into the delivery status of a handful of report packages after it was notified by a doctors' office in late April that three reports had failed to arrive.

Last month, a Canada Post investigation determined some of the medical reports were handed over without signature confirmation and could not be tracked down. Canada Post admits it still doesn't know what happened to the files and is trying to tack them down.

Cancer Care Ontario said it held off announcing that reports had been lost so it could narrow down the number of packages that were actually missing, and it expects to know by next month if the remaining reports cannot be tracked down.

Cavoukian's office is investigating the incident for a possible privacy breach under the Personal Health Information Protection Act.

The commissioner's office checks Cancer Care Ontario's privacy practices every three years, but the last review was in 2008 -- before the agency decided to send the colon cancer results out on paper.