European Union weakens parliamentary supremacy
Parliamentary supremacy is one of the fundamental tenets of the British Constitution, underpinning the legal system and conferring ultimate legislative power on Parliament. Dicey encapsulated this pillar of the Constitution as “the right make or unmake any law whatever…and…no person or body is recognised by the Law of England as having the right to override or set aside the legislation of Parliament".  However, this traditional notion of supremacy has been greatly weakened with the membership of the European Union (EU), seemingly altering the balance of power within the UK Constitution. It can been argued that parliamentary supremacy is now only notionally absolute, no longer resembling the traditional Diceyan conception. Indeed, it may well be that the membership of the EU, along with developments in common law and the introduction of the Human Rights Act 1998, have not just diminished but rather destroyed the supremacy of Parliament altogether.
Case law from the European Court of Justice (ECJ) seems to confirm the decline of Parliamentary supremacy in favour of a more harmonised European pooling of sovereignty. First articulated in Van Gend En Loos  , the idea was clarified in Costa v ENEL  where it was confirmed that “the member States have limited their sovereign rights, and albeit within limited fields, have created a body of law which binds both nationals and themselves".  The wording of the Court seems to allude to an out and out transfer of power rather than a delegation or temporary loan, reflecting the view that membership of the EU has indeed diminished, if not destroyed, the supremacy of Parliament. Repeated in judgments such as Simmenthal v Italian Minister of Finance  , where the ECJ stated that the EU law takes immediate precedence over existing and subsequent conflicting law, the principle of supremacy of EU law has become a fundamental pillar of the European system. According to the ECJ at least, parliamentary supremacy has been replaced by European legislative supremacy.
The terms of the European Community Act 1972 (ECA) are crucial to the analysis of the EU legal order. Section 2(4) provides that all statutes whether already enacted or yet to be enacted must be read and given effect to consistently with enforceable principles of EU law and Section 3(1) elevates decisions of the ECJ to binding precedents for all UK courts and tribunals. Although there is a view that the supremacy of the EU rests on the footing of a domestic statute which could be repealed by simple majority, this does not take into account the practical difficulties of repeal. Furthermore, sovereignty is diminished by the overwhelming scope and breadth of the European treaties such as Maastricht Treaty (1992) which have been continually amended by the treaties of Amsterdam Nice, and most recently by the Treaty of Lisbon in 2009. The growing coverage of these treaties could be seen, in practical terms, as diminishing parliamentary supremacy because they extend the competence of the EU to being able to legislate on a wider area of policy areas.
The application of the ECA in the domestic sphere, especially in relation to conflict between EU and national law has challenged the traditional notion of parliamentary sovereignty. In Factortame (No.2)  the House of Lords categorically accepted the supremacy of EU law and disapplied the Merchant Shipping Act 1988 which directly conflicted with what was then EC law. Lord Bridge stated that “under the terms of the Act of 1972 it has always been clear that it was the duty of a United Kingdom court, when delivering final judgment, to override any rule of national law found to be in conflict with any directly enforceable rule of Community law". 
This seems to destroy any notion of Parliamentary supremacy that may have existed, emphasising, as the ECJ has stated, that EU law is supreme. Indeed, the effect of this statement was deemed “revolutionary"  by Wade, who saw the statement by Lord Bridge as alluding to the fact that Parliament, by passing the ECA, had managed to bind its successors. Traditional theory posits that any Act of Parliament could impliedly repeal an earlier one, by being deemed to override any inconsistent provisions. What this judgment seemed to highlight was, however, that Parliament was bound and that EU supremacy, not parliamentary supremacy, now underpinned the legislative system. For Wade, parliamentary supremacy is ultimately a judicially recognised “political fact" and the decision in this case is the legal revolution through a judicial recognition that, politically, the EU had adopted supreme legislative power.
Attempts to explain away the practical realities of the decision have seem like desperate attempts to retain the idea of parliamentary sovereignty. It has been argued that as the Merchant Shipping Act 1988 was not in direct conflict with EC law so, rather than striking down an Act of Parliament, the Courts were merely interpreting the Act in accordance with Treaty obligations. Proposed by Lord Diplock in Garland v British Rail Engineering  , the conception seeks to retain the idea of parliamentary supremacy, but does so unconvincingly. Only a true and direct challenge by Parliament, through an intentional contradiction of EU law, can reveal the attitude of the Courts to the notion of parliamentary supremacy  . In such circumstances, however, it is unlikely that views such as that taken by Lord Denning’s in McCarthy v Smith  would be accepted. His argument that the Courts would be obliged to apply domestic instead of European law fails to take into account the practical realities of EU integration.
The decision by Laws LJ in Thoburn v Sunderland City Council  , has interpreted the Factortame (No.2)  judgment in another light. Far from destroying parliamentary supremacy, Laws LJ sees the judgment as preserving the power of Parliament to expressly repeal the 1972 Act. In order to do this, he outlined a new set of so called ‘constitutional statutes’ that could not be impliedly repealed, listing, amongst others, the ECA. In doing so, notional supremacy is given back to Parliament. However, in practical terms, the judgment is merely a screen for the realisation that EU membership has indeed diminished the traditional conception of Parliamentary supremacy, even though Laws LJ maintained that “being sovereign, Parliament cannot abandon its sovereignty".  Although it may be argued that “for so long as the Act of 1972 remains on the statute book, as Parliament’s delegates; the law of Europe is not a higher-order law, because the limits which for the time being it sets to the power of Parliament are at the grace of Parliament itself"  this is a fallacy. Practical and political integration with the EU is such that the ECA is not capable of repeal, and so with this, if you accept Wade’s theory that parliamentary supremacy is merely “political fact", die the final notions of parliamentary supremacy.
Although Parliament could be considered supreme in relation to English law, a whole myriad of laws that do not originate in England now form a key element of our legal system. This argument is encapsulated in the newer conception of supremacy outlined by Allan, who states that he “prefer[s] the term 'legislative supremacy', which signifies that Parliament is the supreme law-making body within the UK legal order".  This theoretical explanation broaches the divergent attitudes expressed in the case law by delineating the different spheres of influence that English and EU law now occupy. Commenting on the decision in Thoburn  , Allan goes on to highlight that “the continuing sovereignty and independence of the English legal order remains untouched".  Taken together with his other comments, this helps to preserve the notion of parliamentary supremacy in terms of English law, but accepts that in practical terms, EU law is supreme. This analysis encapsulates a middle ground, a position whereby parliamentary supremacy is no longer expressed to be absolute, but its role as a pillar of the Constitution is not quite ready to be relinquished. 
The membership of the EU has also contributed to the adoption of the Human Rights Act 1998, incorporating the European Convention on Human Rights into domestic law. Although it has been claimed that “the carefully and subtly drafted 1998 Act preserves the principle of parliamentary sovereignty"  , the ability of the Courts to make a declaration of incompatibility in accordance with Section 3 of the Act seems to contradict this argument. This provision allows domestic judges to ascertain whether a national law complies with the Act (and by association, the European Convention), and to declare it incompatible if it falls foul of its provisions. Again, this highlights the influence of an outside element on the supremacy of Parliament. Now, parliament can make and unmake any law it chooses, as long as it complies with Human Rights law- hardly the classic example of supremacy.
In the last few months, the idea of parliamentary supremacy has once again become a focal point for discussion in Constitutional law circles.  With the passage of the EU bill through Parliament, the existence of parliamentary supremacy in relation to the EU legal order has been questioned. With Clause 18 of the European Union bill touted to contain a parliamentary supremacy clause  , William Hague has argued that the clause “confirms and affirms the position that EU law in this country is only recognised by virtue of the authority of acts of Parliament".  Although membership of the EU may be seen to have diminished or destroyed parliamentary supremacy, it is clear that there are plans afoot to resurrect it. One wonders, however, whether this just confirms the status quo that Parliament could indeed expressly repeal the ECA and remove itself from the Union without actually having an impact on the supremacy of the EU legal order. That much is clear already from case law but, noticeably, what the proposed clause fails to do is seek to restructure the hierarchy of legal orders that currently exists.
In conclusion, it is clear that with membership of the EU, parliamentary supremacy is diminished so much as to leave it almost destroyed. The traditional notion espoused by Dicey has long been crushed by the ECA and judgments by the ECJ and House of Lords which confirm the supremacy of the EU legal order. What remains, however, is a notional concept that is desperately retained by those that still assert parliamentary supremacy. Indeed, the draft European Union bill highlights as much. However, seeking to hold on to the phantom of parliamentary supremacy does nothing for it in practical terms, for what was once a pillar of our constitution is no more. To face up to the practical realities of the ECA and the European legal order may take time, but once it is fully recognised, it will be clear that although “under English law nobody has the power to override or to set aside a statute…it is no longer the case that English law is the only law that is applicable in England".  Times have changed, and it is plain that membership of the EU has indeed diminished parliamentary supremacy.