Google, YouTube, Facebook, Twitter and other internet companies must track and remove terrorist content from their platforms within one hour from its referral, according to measures announced by the European Comission (EC).

The 'one-hour rule' is part of a recommending a set of operational measures to be taken by companies and European Member States as the European Union challenges the illegal content online. The specific rule applies to terrorist content only, and not any kind of online illegal content, which is also tackled by the EC's reccommendations.

European governments have said that extremist content on the web has influenced attackers who have killed people in several European cities after being radicalized.

The recommendation, which is non-binding but could be taken into account by European courts, sets guidelines on how companies should remove illegal content generally -- from copyright infringements to hate speech -- and advises a quicker reaction to extremist material.

The EC also called on the technology sector to adopt proactive measures such as automated detection to rid their platforms of illegal content.

These recommendations apply to all forms of illegal content ranging from terrorist content, incitement to hatred and violence, child sexual abuse material, counterfeit products and copyright infringement.

Vice-President for the Digital Single Market Andrus Ansip said: "Online platforms are becoming people's main gateway to information, so they have a responsibility to provide a secure environment for their users. What is illegal offline is also illegal online. While several platforms have been removing more illegal content than ever before - showing that self-regulation can work - we still need to react faster against terrorist propaganda and other illegal content which is a serious threat to our citizens' security, safety and fundamental rights."

The European Commission will monitor the actions taken in response to the recommendations and determine whether additional steps, including, if necessary legislation, are required.