A German consumer rights group said on Monday that a court had found Facebook's use of personal data to be illegal because the U.S. social media platform has not informed users about the treatment of their data.

The Federation of German Consumer Organisations (vzvb) said that Facebook's default settings and some of its terms of service were in breach of consumer law, and that the court had found parts of the consent to data usage to be invalid.

"Facebook hides default settings that are not privacy-friendly in its privacy center and does not provide sufficient information about it when users register," said Heiko Duenkel, litigation policy officer at the vzvb.

"This does not meet the requirement for informed consent."

Facebook said it would appeal. In a statement, it said it had already made significant changes to its terms of service and data protection guidelines since the case was first brought in 2015.

Facebook also plans to update its data protection guidelines and its terms of service so that they comply with new European Union-wide rules that are due to enter force in June.

Germany's competition authorities also said in December that it objected to the way the company gains access to third-party data when an account is opened. This includes tapping information from its own WhatsApp and Instagram products - as well as how it tracks which sites its users access.