Treaty between the Kingdom of Belgium, the Kingdom of Denmark, the Federal Republic of Germany, the Hellenic Republic, the French Republic, Ireland, the Italian Republic, the Grand Duchy of Luxembourg, the Kingdom of the Netherlands, the United Kingdom of Great Britain and Northern Ireland (Member States of the European Communities) and the Kingdom of Spain and the Portuguese Republic concerning the accession of the Kingdom of Spain and the Portuguese Republic to the European Economic Community and to the European Atomic Energy Community