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.

"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.

The break-in was discovered in mid-December, but was kept confidential upon the request of law enforcement officials.

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

Shares of TJX fell 22 cents to close at $29.63 on the New York Stock Exchange.

The company posted advice on checking credit records on its Web site and said concerned customers can call (866) 484-6978 for more information.

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TJX Cos.: http://www.tjx.com