Deutsche Telekom has said that the theft could lead to the disclosure of personal details on some 17 million customers. Among the data compromised is names addresses and phone numbers for customers.
Details on several celebrities, politicians and business leaders are reportedly among the items lost. No customer credit card information or bank details are believed to have been compromised in the theft.
Though the breach occurred in 2006, news of the data loss only came to public light when Der Spiegel magazine found the details being offered by a third party.
Word of the breach comes as less than welcome news to Deutsche Telekom, known as T-Mobile outside of Germany.
Earlier this year, the company came under heat from the German government after Der Spiegel broke the news that the company was using private investigators to spy on journalists and directors who were suspected of leaking information on the company's restructuring plans.