The EU treaties can be amended unanimously by an intergovernmental conference - one member state can therefore block a Treaty change. The final decision must then be sent for ratification by the national parliaments (in some countries by an extended majority) or referenda.
The Lisbon Treaty from 2009 included a "Convention-Method" for treaty amendments in Article 48.2 TEU. Member states, the EP and the Commission are now all able to initiate amendments.
The European Council mayd dcide - by simple majority if- a Convention shall be convened. The Convention decides "by common accord" whether to adopt or not to adopt these amendments. Amendments adopted by the Convention must then have to be ratified by all member states before entering into force.
Furthermore, the Lisbon Treaty foresees a “simplified revision procedure” in Art. 48.6 and 48.7 TEU, the first followed by national ratification, the second simplified method without ratification:
- Revision by the European Council (not IGC nor Convention) by unanimity on a proposal from the Commission, a member state or EP – entry into force after ratification by the member states according to Art. 48.6 TEU.
- General passerelle: European Council moves unanimity to QMV or special to ordinary legislative procedure by unanimity – no ratifications needed, but veto rights of national parliaments ccording to Art. 48.7 TEU.
If a treaty amendment is not carried within 2 years a political solution may be found among prime ministers in the European Council.