President Vladimir Putin on Tuesday signed a law requiring Internet companies to store all personal data of Russian users at data centres in Russia, a move which could chill criticism on foreign social networking sites like Facebook and Twitter. These companies, which do not have offices in Russia, have become a vital resource for opposition groups and refuse to hand over user data to governments. The use of Russian data centres would make them and other Internet companies subject to Russian laws on government access to information. The Kremlin said the law was aimed at improving the management of personal data of Russian citizens on computer networks and that companies which do not comply with the legislation would be blocked.