BOSTON (Reuters) - The man accused of masterminding the largest identity theft in U.S. history agreed to plead guilty to related charges, according to court papers filed in Boston federal court on Friday.

Albert Gonzalez is accused of helping to steal millions of credit card and debit card numbers from major U.S. retail chains, leading to tens of millions of dollars in fraudulent transactions.

A former government informant who is already in jail, Gonzalez, 28, agreed to plead guilty to 19 counts in Massachusetts by September 11. The agreement also resolves charges pending in federal court in New York.

He also faces charges in a New Jersey federal court for a similar crime that is the largest in U.S. history. That case is not affected by the Boston plea, a spokesman at the U.S. District Court in New Jersey said.

A year ago, Gonzalez and 10 other people from five countries were charged with stealing 41 million credit and debit card numbers from retailers TJX Cos, BJ's Wholesale Club Inc, OfficeMax Inc, Boston Market, Barnes & Noble Inc, Sports Authority, Forever 21 and DSW Inc.

Then, on August 17, Gonzalez was indicted yet again, in U.S. District Court in New Jersey, for stealing more than 130 million card numbers from credit-card processor Heartland Payment Systems and retail chains 7-Eleven Inc and Hannaford Brothers Co.

His lawyer, Rene Palomino, did not immediately return calls seeking comment.

When Gonzalez was first charged, authorities said that he drove around Miami with a laptop computer looking to hack into wireless networks at retail stores. When he found an open network, he collected and then sold account numbers, which were used to conduct tens of millions of dollars in fraudulent transactions.

"Albert Gonzalez was at the top of the heap in stealing credit card numbers and with him off the street people can breathe a huge sigh of relief," said Richard Smith, a principal at Boston Software Forensics, which conducts security audits. "This does not mean that the problem is eliminated. There are a lot of other people doing this in a less sophisticated way which is still very dangerous."

Officials at discount retailer TJX and bookseller Barnes & Noble were not immediately available for comment.

The case is U.S. v. Albert Gonzalez in U.S. District Court in Massachusetts, No. 08-10223.

(Reporting by Scott Malone, Ross Kerber, Svea Herbst-Bayliss and Toni Clarke in Boston, additional reporting by Edith Honan in New York, editing by Leslie Gevirtz and Matthew Lewis)