Over the last thirty years, research on the immigration policy-making of liberal democratic states has been preoccupied with the puzzle of why migrant inflows have reached unprecedented levels in Western countries, despite popular calls for restrictionism. A common response of scholars to this puzzle is that whilst governments endeavour to reflect public preferences for restrictive immigration policy, they are prevented from doing so by norms and institutions that are characteristic of liberal democracies. These ‘liberal constraints’ include the national judiciary; international human rights norms; and supranational institutions, such as the European Union. But what of the national legislature? What is the role of this key liberal institution in shaping immigration law within Western democracies? On this question, the literature says remarkably little.

This thesis endeavours to redress this apparent neglect. Its case study is the United Kingdom, which is viewed, on the basis of existing research, as a ‘most-likely’ case for having a weak legislature; and in which the executive branch of government has been shown to be relatively unconstrained by the judiciary in comparison with other European states. Does anything, then, act to constrain the immigration restrictionism of the British government?

Informed by a novel theoretical framework, ‘interpretive political opportunity structures’, the investigation focuses upon the Parliamentary passage of a single policy: the Immigration Bill 2013-14. Its analysis is based on a detailed examination of the Bill and its legislative process; and on thirty-three interviews that I conducted with key immigration policy stakeholders, including two Government ministers, one from each of the Coalition parties; Government and Opposition MPs; members of the House of Lords; civil servants; legal professionals; and lobbyists.

The findings reveal that the UK Parliament had an important liberalising impact upon the Bill, acting to constrain the restrictionist bent of the executive. If the UK is accepted as a case in which we are most likely to find the opposite of this, i.e., a legislature impotent against a dominant executive, then the orthodox view that the legislature is in general a marginal actor in shaping immigration law may have to be revised.

Notably, the unelected upper chamber, the House of Lords, appeared to constitute a stronger check on executive power than the elected lower chamber, the House of Commons. This is consonant with Peers’ understanding of their duty to legislate responsibly, rather than responsively (i.e., in line with popular opinion) like MPs in the Commons. Insulated from populist pressures, the Lords invites comparison with respect to its function and impact to the judiciaries of other Western nations, suggesting, perhaps, that in the British constitutional system, known for its pusillanimous judiciary, the Lords evidences an ‘adaptation’ to the marked power imbalance between the judicial and executive branches of the UK state.