The largest US bookseller, Barnes and Noble, says shoppers may have had credit card data stolen after keypads in 63 stores were tampered with.

The chain said that a "sophisticated criminal effort" had planted bugs on one pad in each store.

By 14 September, Barnes and Noble had disconnected all keypads in its almost 700 stores.

The firm says purchases made on its website and on Nook, its e-reader device, were not affected.

An anonymous Barnes and Noble official told the New York Times that it did not immediately alert customers, at the request of US investigators.

The FBI had asked the book store not to make any announcement until the agency could determine who was behind the attacks.

Unauthorised purchases were made on some credit cards in September, according to the New York Times.

Barnes and Noble said it was continuing to work with the investigation and recommended that customers change their card Pin numbers.

The stores affected were in the states of California, Connecticut, Florida, Illinois, Massachusetts, New Jersey, New York, Pennsylvania and Rhode Island, according to a Barnes and Noble press release.