Millions of credit card accounts may have been compromised after hackers stole customer information last year from computer systems of TJX Cos., the U.S. parent firm of Canadian retailers Winners and HomeSense.

TJX, which is based in Framingham, Mass., confirmed Wednesday that some customer information has been stolen from its systems, but the company said the full extent of the theft and number of customers affected are not yet known.

Citing unnamed sources, the Globe and Mail reported Thursday that two million Visa credit card accounts in Canada and 20 million Visa cards globally have been affected.

It is unclear how many other credit cards may have been compromised.

The hacking was discovered in mid-December. The hackers got into the computer systems of U.S. discount retailer TJX Cos. The systems process and store information about transactions using credit and debit cards, cheques and merchandise returns.

The company said the stolen information covers transactions between 2003 and part of 2006.

“Since discovering this crime, we have been working diligently to further protect our customers and strengthen the security of our computer systems, and we believe customers should feel safe shopping in our stores," Ben Cammarata, chairman and acting chief executive officer of TJX, said in a 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.

TJX operates 184 Winners and 68 HomeSense stores in Canada. It also has 826 T.J. Maxx, 751 Marshalls, 271 HomeGoods and 162 A.J. Wright stores, as well as 36 Bob’s Stores, in the United States. In Europe, the company runs 212 T.K. Maxx stores.