BOSTON (March 29) - More than two months after first disclosing that hackers accessed customers' financial data from its computers, discount retailer TJX  Cos. has revealed that information from at least 45.7 million credit and debit cards was stolen over an 18-month period.

In a regulatory filing that gives the first detailed account of the breach initially disclosed in January, the owner of T.J. Maxx, Marshall's and other stores in North America and the United Kingdom also said another 455,000 customers who returned merchandise without receipts had their personal data stolen, including driver's license numbers.

The data that was stolen covers transactions dating as far back as December 2002, TJX said in the filing Wednesday with the Securities and Exchange Commission.

TJX spokeswoman Sherry Lang did not immediately return a telephone message from The Associated Press seeking comment late Wednesday.

But Lang told The Boston Globe, which first reported the filing Wednesday night, that about 75 percent of the compromised cards either were expired or had data from their magnetic stripes masked, meaning the data was stored as asterisks, rather than numbers.

Lang said the extent of the damage may never be known because of the methods used by the intruder. Much of the transaction data was deleted by TJX in the normal course of business between the time of the thefts and the time they were discovered, the filing said, making it impossible to know how many card numbers were obtained.

"There's a lot we may never know and it's one of the difficulties of this investigation," Lang said. "It's why this has taken this long and why it's been so tedious. It's painstaking."

Avivah Litan, vice president of research and advisory company Gartner Inc., told the Globe the TJX breach is "the biggest card heist ever."

"This was obviously done over a long period of time, in many locations," she said. "It's done considerable damage."

Police charged six people in Florida last week with using credit card numbers stolen from a TJX database to buy about $1 million in merchandise with gift cards.

In Wednesday's filing, TJX said for the first time that Dec. 18, 2006, was the date it first learned that there was suspicious software on its computer system.

TJX said it believes hackers invaded its systems in July 2005, on later dates in 2005 and also from mid-May 2006 to mid-January 2007. The company said no customer information was stolen after Dec. 18, one day before it hired General Dynamics Corp. and IBM  Corp. to investigate. By Dec. 21, those investigators determined that the computer systems had been breached and that an intruder remained on the systems.

TJX said it notified federal authorities Dec. 22, and on Jan. 3, TJX officials and Secret Service agents met with banks and payment card and check processing companies to discuss the computer intrusion.

The company issued a news release Jan. 17 disclosing the breach but did not say how much data was stolen.

Framingham-based TJX is facing an investigation by the Federal Trade Commission and lawsuits from individuals and banks accusing it of failing to do enough to safeguard private data and of delaying disclosure of the problem.

The company said in Wednesday's filing that its forensic investigation of the intrusion is ongoing and it is continuing to work to strengthen and protect its computer systems.