Raleigh, N.C. — The American Civil Liberties Union of North Carolina is telling the state Department of Revenue to back off on a request for "constitutionally protected private information" of Amazon.com customers.

In a letter Thursday to Revenue Secretary Kenneth Lay, the group says it will join an existing lawsuit brought by the online retail giant if the department "persists in its demand" for North Carolina customers' names and addresses.

"If the department persists in its demand that Amazon now additionally provide detailed user information … the constitutional rights of our clients and tens of thousands of North Carolina consumers will be violated," the letter states.

Amazon filed suit in federal court last month to block the department's attempt to get the information of those who bought or received items from the company since 2003.

"The ACLU is not taking issue with the department’s authority to collect taxes on the value of these purchases, but there is no legitimate reason why government officials need to know which North Carolina residents are reading what books or purchasing which specific brands of products," Jennifer Rudinger, executive director of the ACLU of North Carolina, said in a statement.

According to its lawsuit, Amazon has already provided the revenue department with data about the purchases, including product codes that reveal the exact items purchased.

State revenue officials have called the lawsuit "misleading," saying they never asked the retailer to turn over detailed information that would reveal personal preferences.

Lay re-emphasized in a statement late Thursday afternoon that the department is not interested in consumers' buying habits.

"The department does need a general product description – for example, "book" or "food." That description is necessary to determine the rate of tax, because different items are taxed at different rates," he said. "The department does not require the identification of customers' choices of expressive material."

The information would cover about 50 million purchases, Amazon has said.

Because Amazon has no offices or warehouses in North Carolina, the company isn't required to collect the customary sales tax on shipments.

North Carolina requests voluntary compliance from taxpayers, asking them to include a "consumer use tax" on their individual income tax returns for anything purchased or received through the mail.

By some estimates, the state loses $140 million a year in unpaid taxes from online sales.