A hacker has stolen the personal details of around two million Vodafone Germany customers.
They gained access to one of the mobile phone operator’s servers and obtained names, addresses and bank account numbers.
Vodafone says the hacker did not get hold of passwords and security numbers.
But the company warned customers that criminals could launch so-called “phishing” attacks, using fake emails, to try to trick them into revealing more details.
The investigation is understood to be looking into a person who was working for a sub-contractor for Vodafone’s administration system.