Vodafone has revealed that the private details of over two million customers including names, bank account numbers, addresses and birth dates have been stolen by hackers.
The extensive information was taken from a database held inside the firm's internal network.
The company said that all customers have been informed of the breach and it has advised them to "be very careful when asked over the phone or via email to give out personal information like passwords or credit card details".
Although the stolen details could be used in phishing attacks and to create fake online accounts, the cyber criminals should not be able to use the data obtained to access bank accounts, Vodafone said in a statement.
Credit card details, PIN numbers, passwords and telephone numbers were not accessed by the hackers.
"Vodafone deeply regrets the incident and apologises to all those affected," the firm said, adding: "This attack could only be carried out with high criminal intent and insider knowledge and was launched deep inside the IT infrastructure of the company."
Vodafone Germany has around 36 million customers.
A suspect has now been charged and had his house searched. It has not been revealed when the attack took place, with the company saying that victims could not be informed whilst the investigation was ongoing.