On the surface, at least, parliamentary sovereignty — a phenomenon that applies to the UK, or Westminster, Parliament, but not to the UK’s devolved legislatures — is a simple concept. To paraphrase Dicey, Parliament has the legal authority to enact, amend or repeal any law, and no-one has the legal authority to stop it from doing so. But this notion is as extravagant as it is simple: it means, as Stephen famously put it, that a law directing the killing of all blue-eyed babies would be valid. The fact that such laws remain unenacted is thanks to “political constitutionalism” as opposed to “legal constitutionalism”: it is political, not legal, factors — including, one hopes, legislators’ own sense of morality — that operate as the restraining force.
It is often assumed that the sovereignty of Parliament follows from the absence in the United Kingdom of a written constitution, the existence of such constitutions generally being associated with legislatures that have only limited powers. Within such legal systems, the written constitution usually performs two functions (which are in truth flip sides of one coin) that rule out anything like parliamentary sovereignty. The constitution confers authority on the legislature; and the constitution restricts the legislature’s authority (by omitting to confer the power to do certain things). Within this type of constitutional framework, the legislature only has those (limited) powers that the constitution grants: and if the legislature attempts to make laws beyond the powers granted to it, then (often) courts can intervene by quashing unconstitutional legislation (or simply refusing to apply it, on the ground that it is not really “law”).
In the UK, however, in the absence of a written constitution, there is nothing to tell us what powers Parliament has: and there is equally nothing to tell us what powers (if any) Parliament lacks. It appears, therefore, that the constitution fails to perform the twin functions — of allocating and limiting authority — that usually result in something other than legislative sovereignty. But while this seems to follow from the absence of a written constitution, it does not necessarily follow. The fact that there is no written constitution performing the functions mentioned above does not automatically mean that there is no constitution performing those functions.
In other words, it is conceptually possible for an unwritten constitution to ascribe power to — and circumscribe the power of — the legislature. The constraining capacity of a constitution derives not from the fact that it is written; rather, it derives from the fact that it enjoys a legal status superior to that of regular law, with the result that enacted laws are valid only to the extent that they respect the terms of the legally superior constitution. The question then becomes whether — for all that it is unwritten — the UK’s constitution may enjoy the kind of legal superiority more readily associated with textual constitutions, such that it — like its written counterparts — may claim some sort of constraining force in relation to the legislature.
There is no definitive answer to this question; what evidence there is is circumstantial. The ability of Parliament to enact law — and its claim to unconstrained law-making capacity — is rooted in the constitutional settlement reached in the aftermath of the Glorious Revolution at the end of the 17th century. Unusually, however, that settlement was never cemented by means of being recorded in a superior constitutional text. It is for this reason that writers such as Wade argue that the sovereignty of Parliament is ultimately secured not by law, but by a “political fact”: a consensus that emerged in the wake of the Glorious Revolution and that remains in place unless and until it breaks down as a result of a “technical legal revolution”.
There is a certainly a good deal of evidence to suggest that that consensus — and so the sovereignty of Parliament — remains in place today; no judgment of a UK court specifically rejects the notion of parliamentary sovereignty. That said, there are dicta, perhaps most notably in Jackson v Attorney-General, that call into question the idea that Parliament has unlimited legislative power. There are, for instance, suggestions in that case that if Parliament were to attempt to remove the courts’ powers of judicial review, the courts might refuse to recognise such legislation as valid. This implies — as, perhaps paradoxically, Wade himself suggested — that judicial review is a “constitutional fundamental” that even Parliament cannot disturb.
However, it would be a very brave judge who actually went through with threats of the type made — or at least hinted at — in Jackson. Without a written constitution, there is no roadmap that tells us what would happen if a court were to refuse to apply an Act of Parliament: and it cannot be taken for granted — particularly if the legislation concerned were generally popular, albeit perhaps oppressive to a minority — that the courts would prevail. Nor, in the absence of a written constitution, do judges have the luxury of a text that identifies fundamental constitutional values and legitimises judicial protection of them. Judges who were to enforce such values over and above democratically enacted legislation would thus find themselves in a very exposed position.
It is unsurprising, therefore, that courts do not seek out conflict with Parliament, preferring instead to confer a degree of protection on fundamental constitutional values by interpreting legislation — in cases like Anisminic and Witham — consistently with them, rather than refusing to apply it on the ground that it infringes such values. This sort of interpretative approach, of course, must have its limits: if legislation is sufficiently explicit, then there is little, if any, room for interpretative manoeuvre. However, just as courts are not eager to provoke a constitutional crisis, so Parliament is not anxious to do so. As a result, both sides, for the most part, exercise a degree of self-restraint born of healthy concern as to how the other might react in the event of an excessive use of legislative or judicial power. It is this sort of constructive institutional tension — together with the restraining effect of democratic politics — that forms the context in which the practical significance of parliamentary sovereignty falls to be understood. It follows that even if we accept the Diceyan orthodoxy that Parliament possesses unlimited legislative power, this does not mean that Parliament is in a position to exercise the full width of that authority.
- Elliott and Thomas, Public Law (OUP 2014), 2nd edition, pp 208-232
- Elliott, “Parliamentary sovereignty in a multidimensional constitution” (2013) Public Law For Everyone
- Wade, “The Basis of Legal Sovereignty”  Cambridge Law Journal 172
- Jackson v Attorney-General  UKHL 56
- Anisminic Ltd v Foreign Compensation Commission  2 AC 147
- R v Lord Chancellor, ex parte Witham  QB 575