This chapter analyzes engagement and disengagement with international institutions from the perspective of U.K. law. The first part of the chapter considers the relevant legal rules that pertain to engagement by the United Kingdom in international institutions. It is divided into three sections. The first section is directed toward dualism as understood in U.K. constitutional law, whereby an international treaty cannot take effect in national law unless it has been transformed or adopted into domestic law, thereby preventing the executive from undertaking obligations without the imprimatur of the U.K. legislature. The second section explains the U.K. constitutional rules designed to prevent the executive from ratifying an international treaty, and hence committing the United Kingdom at the international level, before Parliament has had the opportunity to consider the treaty. This area is interesting, since it reveals a shift from practice, to a convention, and then to a statutory obligation. The third part investigates the limits of dualism, connoting in this respect that the doctrinal rules explicated here apply to formal treaties, but do not cover all global regulatory rules, which can impact, de jure or de facto, on the United Kingdom. The focus in the second section of the chapter shifts to the constitutional constraints that limit the national applicability of a treaty regime that the United Kingdom has ratified. Parliament may impose constraints on delegation, which condition the legal reception in U.K. law of changes made by an international organization. There are, in addition, constitutional constraints fashioned by the courts, which can affect the acceptance of rules or decisions made by an international organization, to which the United Kingdom is a party, within the U.K. legal order, more especially where U.K. courts feel that such a rule of decision can impact adversely on U.K. constitutional identity. These judicially created constraints can be interpretive or substantive. The final part of the chapter is concerned with disengagement from international institutions. The relevant legal precepts are, to a certain degree, symmetrical with those that govern initial engagement. The basic starting point is that the executive, acting pursuant to prerogative power, negotiates withdrawal or disengagement from an international organization, and Parliament then enacts or repeals the requisite legislation to make this a legal reality in national law. Matters can, however, be more complex, as exemplified by the litigation concerning the United Kingdom’s exit from the European Union.
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