The French data protection authority, CNIL, has declared the use of Google Analytics illegal due to the transfer of personal data to the US. This decision follows a similar ruling in Austria and is based on the European Union's General Data Protection Regulation (GDPR). The CNIL found that Google Analytics assigns a unique identifier to each visitor, which constitutes personal data and is transferred to the US. This transfer is not considered to provide an adequate level of protection for European data.