The Member States 25 March 1957, Consolidated version of the Treaty on the Functioning of the European Union#PART THREE - UNION POLICIES AND INTERNAL ACTIONS#TITLE X - SOCIAL POLICY#Article 155(ex Article 139 TEC)

4. The provisions adopted pursuant to this Article: — shall not affect the right of Member States to define the fundamental principles of their social security systems and must not significantly affect the financial equilibrium thereof, — shall not prevent any Member State from maintaining or introducing more stringent protective measures compatible with the Treaties. 5.