The company has reported the incident to the police Computer Crime Unit.
The hacker published the user names and passwords of thirty Belgacom Internet customers on the French-language website adsl-bc.org.
The hacker says that he/she did so to put pressure on Belgacom to up the current 25 gigabyte per month limit it imposes on its broadband internet customers.
He/she added that the user names and pass words of many thousands of other Belgacom customers would be published if his/her demands were not met.
Belgacom has informed the affected customers and advised them to change their passwords and user names.
The company stresses that the information published could only be used to access the customers’ email accounts.