TJX Companies said 45.7 million accounts were compromised over nearly a two-year period, in an update of an investigation into a data breach of customer records.

The scope of the breach, which was initially disclosed in January, is far wider than previously believed.

Avivah Litan, an analyst with research company Gartner, said: "This is the largest security breach we've ever had worldwide. There was a case at CardSystems where 40 million records were exposed but this one looks like it was a case where the information was stolen."

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