Target says up to 70 million customers had payment card and personal data stolen from the company's databases in December - 30 million more that it first thought.
The giant US retail company said credit card numbers, names, postal addresses, phone numbers and email addresses were taken.
The BBC reports the data breach began on or around Black Friday on 29 November, one of the busiest shopping days of the year.
Target said the data was stolen between Thanksgiving and 15 December.
In December, security researcher Brian Krebs said data-stealing code had been installed on card-swipe machines at tills in all 1797 Target stores.