The Council, acting unanimously on a proposal from the Commission and after consulting the European Parliament, shall lay down provisions relating to the system of own resources of the Community, which it shall recommend to the Member States for adoption
in accordance with their respective constitutional requirements.
The amendments shall enter into force after being ratified by all the Member States in
accordance with their respective constitutional requirements.
It is true that Article 50, unlike Article 49 for accession treaties, does not explicitly require a «ratification by all the contracting States in
accordance with their respective constitutional requirements».
It is also likely to require ratification by all the Member States in
accordance with their respective constitutional requirements («mixed agreement»), except if it only contains provisions falling within the scope of the exclusive competences of the European Union (Common Commercial policy for example).
The agreement would then require the unanimous approval of the Council, in addition to the approval of all Member States «in
accordance with their respective constitutional requirements» (Article 218 (8) TFEU).