Recent executive conduct in the United States under Donald Trump has revived an old constitutional anxiety: what happens when those at the apex of executive power are willing to test, stretch, or disregard the limits that law and convention are supposed to impose? While the details of US constitutional law are distinctive, the underlying concern is not. It is one the UK constitution must also confront. What limits would actually constrain a Prime Minister determined to behave in a similar way to the recent actions of Donald Trump in the international arena?
This is not a question about importing American constitutional controversies into a British setting, nor about hypothetical bad actors. It is a structural question about whether the UK’s constitutional arrangements operate as genuine constraints on executive authority, or whether they depend too heavily on good faith, political culture, and self-restraint. The answer matters because the UK constitution is often praised for its flexibility. But flexibility is not synonymous with resilience. A constitution that adapts easily may also accommodate overreach more readily than is often assumed.
Executive authority, prerogative and political convention
The UK constitution is not a codified document that clearly defines executive power and its limits. Authority is instead distributed across statute, common law, and the royal prerogative. In routine circumstances, this arrangement functions tolerably well. In moments of constitutional stress, however, its vulnerabilities become more visible.
Nowhere is this clearer than in foreign affairs, national security, and the use of force. These domains remain primarily governed by prerogative powers exercised by ministers on behalf of the Crown. While Parliament has asserted a stronger political role since the Iraq War, particularly through debates and retrospective approval, this role has never been legally entrenched. The government itself acknowledged in the 2011 Cabinet Manual that parliamentary approval for military action operates as a political convention rather than a legal requirement.
The consequence is constitutionally significant. A Prime Minister may lawfully deploy military force abroad without prior parliamentary consent. Parliament may object, scrutinise, or criticise, but it cannot legally prevent such action in advance. This produces a constitutional structure in which the most coercive exercises of state power remain subject primarily to contemporary political control rather than enforceable legal limits.
This is not merely a theoretical concern. The constitutional position creates a permissive environment for quick action and subsequent justification, particularly where the executive controls intelligence, timing, and information. A Prime Minister inclined to frame an exceptional act as urgent, necessary, or lawful would encounter few formal legal barriers to doing so.
Law as justification rather than constraint
A recurring feature of contemporary constitutional stress has been the transformation of legal language from a constraint on power into a vocabulary for legitimising its expansion. Exceptional actions are described as routine, coercive force is reframed as enforcement, and constitutional limits are rhetorically acknowledged while substantively bypassed.
The UK is not immune to this phenomenon. The courts have repeatedly emphasised that prerogative powers are subject to legal limits, most notably in Miller (No 1) and Miller (No 2). Yet those cases also demonstrate the limits of judicial constraint. Courts intervene selectively, cautiously, and often after the political moment has passed. As Alison Young has observed, judicial review in the UK frequently operates as an ex post accountability mechanism rather than an ex ante barrier, especially in areas touching high policy.
Even after Miller (No 2), significant areas of executive authority remain insulated from effective judicial control. Decisions involving the deployment of force, diplomatic recognition, and extraterritorial action continue to attract a high degree of deference. Courts are reluctant to adjudicate questions framed as matters of national security or foreign relations, and even where jurisdiction is asserted, remedies are often limited.
The result is a constitutional environment in which a Prime Minister willing to accept legal risk may reasonably calculate that judicial intervention is unlikely to prevent action in real time. Law remains relevant, but primarily as a tool of retrospective evaluation of action toward foreign actors rather than as an immediate restraint.
Parliament, political accountability, and the limits of convention
Parliament is often described as the primary constitutional check on executive power. In principle, this is correct. In practice, though, its constraining force depends heavily on political conditions. Where a government commands a substantial majority, parliamentary scrutiny can become expressive rather than determinative.
Debates may be held, select committees may investigate, and ministers may be questioned, but the executive retains significant control over timing, framing, and access to information. As the House of Lords Constitution Committee has repeatedly warned, reliance on convention rather than law leaves constitutional safeguards vulnerable to erosion during periods of political stress.
Take the War Powers Convention, which illustrates this problem starkly. Under this convention, the government undertakes to seek parliamentary approval before committing UK armed forces to significant overseas military action. Yet the convention has no legal force and admits of exceptions, particularly where the executive claims urgency or the need for operational secrecy. A Prime Minister determined to bypass Parliament could therefore do so lawfully, accepting political criticism as the price of action. As noted by Ivor Jennings, conventions cannot compel compliance; they function through shared understandings of constitutional propriety, and those understandings lose their constraining force once treated as optional rather than obligatory.
This reveals a deeper structural feature of the UK constitution beyond that which is obvious with regard to international action. Many of its most important safeguards are political rather than legal, cultural and not enforceable. That does not make them insignificant, but it does make them fragile.
A UK hypothetical: conventions under pressure
What makes these concerns more than abstract is how easily they translate into a plausible UK scenario. Suppose a Prime Minister authorised a limited but coercive overseas operation – perhaps framed as the enforcement of international criminal law, sanctions compliance, or counter-terrorism cooperation – without prior parliamentary approval. The action is justified as urgent, lawful, and necessary. Parliament is informed only after the fact, with key details withheld on national security grounds.
What accountability mechanisms would realistically come into play?
Parliament could debate the decision and summon ministers to account, but it could not undo the action. Select committees might investigate, but often only once the political moment has passed. A vote of no confidence would be available in theory, but in practice would depend on party discipline and political incentives rather than constitutional principles. The courts would be unlikely to intervene in real time, and may well regard the matter as non-justiciable or unsuitable for effective remedy.
In other words, the system would rely almost entirely on political restraint and retrospective scrutiny. None of the relevant conventions – the War Powers Convention, ministerial accountability, or respect for parliamentary scrutiny – would provide a legal barrier to action. They would operate, if at all, as mechanisms for criticism rather than constraint.
This is not to suggest that such behaviour would be cost-free. Political consequences might follow. But the constitutional point is sharper: the UK’s safeguards in this area are largely reactive. They assume that the executive is concerned about reputational damage, parliamentary disapproval, or future electoral consequences. Where those assumptions weaken, the constitution offers limited means of immediate resistance.
Why this matters now
It would be wrong to suggest that the UK is on the brink of authoritarianism, but constitutional systems should be assessed based on how they perform under stress, not on how they function in benign conditions.
Periods of international instability, security threats, and populist politics place particular pressure on executive authority. When executives invoke necessity, urgency, or legality to justify exceptional action, constitutional limits are tested. The risk is not merely the action itself, but the precedent it sets and the language used to justify it.
If, as has increasingly been the case abroad, law becomes primarily a vocabulary for explaining power rather than constraining it, constitutionalism is weakened, whether abroad or in the United Kingdom. If restraint depends predominantly on goodwill, it cannot withstand sustained bad faith.
Conclusion
The most troubling question raised by recent executive behaviour internationally is therefore not whether a UK Prime Minister could legally behave in a similar way, but whether the UK constitution is adequately equipped to respond if they did.
The answer, at present, is an uneasy one. The UK constitution contains important principles of legality, accountability, and restraint, but many of the mechanisms that give those principles practical force remain political rather than legal. In areas such as the use of force and overseas executive action, restraint depends heavily on convention, good faith, and political cost.
If that reliance is to be taken seriously, then it also needs to be acknowledged as a vulnerability. One response would be to strengthen Parliament’s role through clearer statutory requirements for approval or notification. Another would be to clarify the scope of judicial review in relation to prerogative powers, not to invite courts into political decision-making, but to ensure that legality operates as a meaningful constraint rather than a retrospective commentary.
More fundamentally, however, this is a question of constitutional attitude. A system that treats conventions as optional and legality as justificatory, rather than restraining, is one that invites erosion over time. If the UK wishes to maintain a constitution that genuinely limits executive power, it cannot rely indefinitely on assumptions of restraint. It must decide where the law should do more of the work that convention currently performs.
The real lesson, then, is not that the UK is constitutionally fragile, but that constitutional resilience requires active maintenance. Exceptional executive behaviour tests not only the character of office-holders, but the seriousness with which a constitution treats its own limits.
Darryn Nyatanga.
Dr Darryn Nyatanga is a constitutional law scholar whose research focuses on executive power, devolution, and constitutional change in the United Kingdom. He holds a PhD from the University of Liverpool.
The Constitution Society is committed to the promotion of informed debate and is politically impartial. Any views expressed in this article are the personal views of the author and not those of The Constitution Society.