The Westminster parliament is sovereign. As a result, the UK is almost unique in not having a codified constitution with entrenched provisions. Parliament can enact legislation on any subject matter it likes, but it cannot bind its successors. Hence the ease with which the failed attempts to achieve a two-thirds majority requirement for an early parliamentary general election under the Fixed-term Parliaments Act 2011 could be overridden by the Early Parliamentary General Election Act 2019 when a simple majority of MPs agreed to pass it in October 2019.

The two Miller cases make it clear that courts can be used to protect parliament’s powers. The second of these also clarified that parliamentary sovereignty means parliament is more constitutionally important than the government.

In Miller I, the Supreme Court was asked whether the ‘prerogative power’ – i.e. the power of the Prime Minister, in this case over foreign relations matters – included a power to notify the EU of the UK’s intention to withdraw. The court concluded that the prerogative power did not include an ability to modify domestic law – notably the European Communities Act (ECA) 1972, which took the UK into the EU at a domestic level. Nor did it include a power to frustrate legislation, or to remove domestic rights. The court’s decision thus reinforced parliament’s role in the constitution. Its first immediate result was that primary legislation was needed to empower ministers to trigger the Article 50 process. This provided parliament with the opportunity, should it have wished to do so, to set conditions on the exercise of this power.

Second, Miller I implied that parliament was more important than the executive. The government's use of prerogative powers could not frustrate the will of parliament as expressed in legislation. This raises important issues as to the nature of a sovereign parliament. The Westminster parliament (‘the legislature’) consists of the Commons, the Lords and the monarch. Most government ministers sit in the Commons, usually supported by their backbench MPs, as do opposition MPs. Ministers may also be selected from the House of Lords. When we refer to the sovereignty of the Westminster parliament, should this mean the sovereignty of the legislature, more particularly the Commons, or of the government?

Whitehall and Westminster

David Howarth refers to this as a contrast between the Whitehall and the Westminster vision of UK democracy. The Whitehall vision places the Crown (whose powers are in practice exercised by ministers) at the centre of the constitution, which favours a strong government. Under this vision, the role of MPs is to shore-up the government, or to propose an alternative government.

The Westminster vision instead focuses on the role of parliament. Under this vision the legislature, in particular the Commons but also the Lords, holds the balance of power and is the final arbiter on policy. Its job is to scrutinise legislative provisions and hold the government to account for its actions.

In autumn 2019, the tension between these two visions played out on both the political and the legal stages. There were moments when the government lost its control over parliamentary time. Standing Order No. 14 formally prioritises governmental business. But these events demonstrated that when faced with a combination of a minority government, a Speaker willing to favour the powers of the Commons over those of the government, and an issue that both causes division within and across parties and is so important that MPs are willing to risk their careers, the legislature can wrestle control of time from the government.

In September 2019, in the shadow of a proposed prorogation (suspension) of parliament, a combination of opposition and backbench MPs was able to initiate legislation that progressed through the Commons in one day. This legislation required the Prime Minister to do something to which, at the time, he was adamantly opposed – ask for an extension to the Article 50 negotiation period (unless the Commons quickly voted in favour either of the Withdrawal Agreement, or of leaving the EU with no deal – which it failed to do).

The attempted five-week prorogation of parliament led to the second Miller case – the combined appeal from Gina Miller’s legal action in the English courts and Joanna Cherry MP’s legal action in the Scottish courts. Unanimously, the Supreme Court quashed the prorogation order. It concluded that the common law placed limits on the scope of the prerogative (i.e. government’s) power of prorogation, and that these limits had been transgressed.

What will happen to parliamentary sovereignty now?

The Supreme Court relied on the constitutional principles of parliamentary sovereignty and parliamentary accountability. Both support a Westminster vision of democracy. The Court indicated that parliamentary sovereignty implies that, in a partnership that involves the legislature and the executive, the legislature is the senior and the executive the junior partner. Parliament is supreme, not the executive. Parliamentary accountability means that the government is accountable to parliament; parliament, in turn, is accountable to the people.

This judgment was a strong assertion of the court’s role in protecting the UK constitution – but, importantly, through upholding the powers of parliament. The Court’s view was that any use of the prerogative power of prorogation potentially undermines parliamentary sovereignty and parliamentary accountability. But the courts will intervene only when the breach of these principles is sufficiently serious; and if it does so, the government can set out its justification. In Miller II, no reasons were provided for proroguing parliament for five weeks of the then eight remaining weeks to exit day and so the prorogation was found unlawful.

Nevertheless, the Prime Minister, while complying with the judgment, asserted that the courts had gone too far. Prorogation, he said, was purely a political matter. But without parliament having enacted legislation to limit prorogation it remains an act of the executive alone. Moreover, it is hard to see how the Commons can hold the executive to account for its decision to prorogue parliament. Any prorogation is merely communicated to parliament. Parliament then cannot hold the government to account if it is no longer sitting.

What will happen to parliamentary sovereignty after the December general election? A Prime Minister with a strong Commons majority seems to favour a Whitehall vision of democracy. The European Union (Withdrawal Agreement) Act 2020 demonstrated a move in that direction, removing parliamentary oversight over the future trade deal. The promised repeal of the Fixed-term Parliaments Act might see a return to an unlimited governmental discretion to dissolve parliament. Other constitutional reforms may further limit the powers of the courts to protect key constitutional principles – possibly including parliamentary sovereignty itself.

Dicey saw parliament as sovereign because, while it enjoyed legal sovereignty, the people enjoyed political sovereignty. He was suspicious of party politics that could weaken the direct accountability of parliament to the people. In a Westminster democracy, the government is the Prime Minister and the Cabinet. Whilst they may be formed from the political party with the largest majority, the electorate as a whole does not vote specifically for a particular government. Hence the deeper question – to what extent is sovereignty of the UK parliament equally justified in a Whitehall, as opposed to a Westminster, vision of democracy?

This article is from the Parliament and Brexit report.