BOSTON -- Shoppers at several popular stores need to keep an eye on their bank statements because their personal information may have been stolen.

TJX Cos., operator of T.J. Maxx and Marshalls discount stores, said Wednesday its computer systems were hacked late last year and customer data has been stolen.

The company said the full extent of the intrusion is not yet known, but it is conducting a full investigation.

The hackers broke into a system that handles credit and debit card transactions, as well as checks and merchandise returns for customers in the U.S., Puerto Rico, and may also involve customers of T.K. Maxx stores in the U.K. and Ireland.

NewsCenter 5's Steve Lacy reported that the thefts happened in December but were not revealed until Wednesday. They could affect millions of consumers around the world. Based in Framingham, the company operates more than 2,500 outlets in the U.S. and abroad. The company said it has identified "a limited number of credit and debit card holders whose information was removed from its system."

The company said another potentially larger groups' information may have been accessed but at this point the full extend of the breach is not known.

"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," TJX Chairman Ben Cammarata said in a statement.

Internet security experts said Internet-based crimes are on the rise.

"The total loss in the U.S., to U.S. industry from Internet crime, is over $400 billion," said Stephen Orenberg of Kaspersky Labs.

The data breach at TJX follows similar breaches at other big companies, such as DSW Shoe Warehouse and Circuit City. Several banks in Massachusetts have been notified and TJX said it has also notified all of the major banks and credit card companies that process transactions and that it's there responsibility to notify customers.

Cracking down on crimes such as these has proven difficult.

"Part of it is because of the sheer numbers. Last year we saw about 80,000 of these new types of criminal viruses," Orenberg said.

TJX said it has hired General Dynamics Corp. and IBM Corp. to upgrade its security system.

NewsCenter 5's Janet Wu reported that T.J.Maxx customer Lynette Boldrighini said she discovered $900 in fraudulent charges on her credit card after she went shopping.

"When I got my credit card bill on Jan. 2, I went through the transactions and I noticed that there were 23 that I did not charge," she said.

Boldrighini said she canceled her credit cards immediately but didn't know the source of the problem until she saw that TJX's system had been hacked.

"They didn't want to tell me if it was really was. Nobody wanted to tell me anything about it, but it seems like it. It might be," she said.

But TJX insisted that the company has no knowledge of any customer information being used illegally.

"I was relieved because then I knew where it had happened because I could not figure out how they had gotten my number," Boldrighini said.

NewsCenter 5's Susan Wornick offered some advice to consumers who have been the victim of identity fraud.

For credit cards -- Visa, MasterCard, American Express or Discover -- there are federal laws that protect consumers. Consumers are not responsible for fraudulent charges. But consumers are responsible for reporting them to their credit card company. The charge will be removed from the account while the investigation is pending.

If a debit card is affected, federal law said consumers might be responsible for the first $50. But bank policies differ, so consumers should check with their bank, Wornick said.

TJX said it will help consumers get free credit reports.

TJX set up a hot line number for concerned customers at 866-484-6978.