The U.S. parent company of Canadian retailers Winners and HomeSense says its computer systems were hacked late last year, exposing the personal information of millions of customers.

About 20 million people shop at the stores, including 2 million Canadians, meaning the retail security breach was one of the biggest in North American history.

"There have been events of about this size but not really much bigger," Richard Reiner, of Telus Security Technology, told CTV News.

Ben Cammarata, chairman of U.S. parent company The TJX Companies, Inc., acknowledged the breach in a press release.

"Our first concern is the potential impact of this crime on our customers, and we strongly recommend that they carefully review their credit card and debit card statements and other account information for unauthorized use," he said.

Portions of the information regarding credit and debit card sales transactions during 2003 and from the period from mid-May to December 2006 may have been accessed in the intrusion, says the release.

The system that was hacked handles transactions along with cheques and merchandise returns in the U.S., Canada and Puerto Rico. Other regions that could be affected include the U.K. and Ireland.

"It is believed that thieves may have stolen credit card and debit card information for as many as two million Canadian shoppers," ROBTv's Michael Kane said Thursday.

The RCMP and their American counterparts told TJX, operator of T.J. Maxx and Marshalls in the U.S., to not say anything publicly until now so that officials could continue the investigation.

The RCMP was notified on Jan. 16 by the U.S. Secret Service about a possible breach.

The investigation is being carried out in the U.S. by the Secret Service, but local Canadian police services of jurisdiction will provide help if asked, says the RCMP.

The company said the full extent of the breach is still unknown.

"Sources close to VISA say worldwide as many as 20 million credit cards may have been compromised," said Kane.

The company has hired General Dynamics Corp. and IBM Corp. to help upgrade their security systems.

"It's prudent on the part of a consumer to remain vigilant with respect to what's being charged to one's credit cards," Richard Reiner, chief technology officer at Telus Security Solutions told CTV Newsnet Thursday.

"(But) I think the majority of the responsibility for incidents and issues like this (is) on business rather than on individuals."

TJX operates 826 T.J. Maxx, 751 Marshalls, 271 HomeGoods, 162 A.J. Wright and 36 Bob's Stores in the United States. In Canada, the Company operates 184 Winners and 68 HomeSense stores and in Europe it operates 212 T.K. Maxx stores.

With a report by CTV's Graham Richardson and files from The Associated Press