1. Introduction

The United Kingdom, as the origin of modern representative government, and Japan, an island nation geographically distant yet institutionally influenced by Britain, share substantial similarities in their political systems while also displaying nuanced differences. Both countries implement a parliamentary system and uphold a constitutional monarchy, yet the relationships among the parliament, the cabinet, and the judiciary vary in subtle but important ways. This paper focuses on analyzing the differences between the two political systems.

2. Divergent Roles of the Monarch

As the birthplace of modern constitutional monarchy, the United Kingdom is widely regarded as a quintessential constitutional monarchy. With the evolution of constitutional principles, the traditionally feudal nature of monarchical power has gradually faded. In the absence of a written constitution, the powers and responsibilities of the British monarch are defined through a combination of laws, conventions, and traditions. Although the monarch remains the head of state, most powers have been transferred to Parliament and the Cabinet through constitutional conventions. The monarch’s primary functions include appointing the leader of the majority party in the House of Commons as Prime Minister, approving legislation, dissolving Parliament and calling general elections upon the Prime Minister's advice, and representing the country in diplomatic activities such as treaty signings and meetings with foreign heads of state. In theory, the monarch still retains certain "Royal Prerogatives," such as declaring war and peace, appointing government officials and diplomatic envoys, and conferring honors. However, these prerogatives are, in practice, exercised by the Prime Minister and ministers, who are accountable to Parliament and the electorate, rather than to the monarch.[1] The monarch’s role is largely symbolic and ceremonial, with no involvement in policymaking and a strict adherence to political neutrality.

The British monarchy, with its long-standing history, is a vital component of the national identity. As a symbolic figure, the monarch fosters unity and a sense of identity, embodying the country's traditions, stability, and continuity [2]. The monarch presides over the State Opening of Parliament, delivers the Queen’s (or King’s) Speech, and participates in national celebrations and religious ceremonies, serving as a visible representation of the nation. Furthermore, royal family members engage in charitable work and community activities, demonstrating the monarchy’s social value. The British monarchy places great emphasis on tradition and historical continuity, which grants it enduring symbolic significance in the modern political system.

In contrast, Japan’s monarchy underwent a profound transformation following World War II. Under Allied occupation, Japan adopted a system inspired by the British model and promulgated the Constitution of Japan in 1947. Article 1 of the Constitution explicitly defines the Emperor as “the symbol of the State and of the unity of the People,” stripped of all political power. The Emperor’s duties are strictly limited to ceremonial functions, and all actions must be performed “with the advice and approval of the Cabinet.”[3] These include convening the National Diet, promulgating laws and treaties, appointing officials, and conducting national ceremonies such as enthronement rituals, bestowing honors, and receiving foreign envoys. Unlike the British monarch, the Japanese Emperor has no decision-making power under any circumstances and is constitutionally barred from expressing opinions on laws or policies. Article 4 of the Constitution of Japan prohibits the Emperor from intervening in government affairs, ensuring the Emperor’s complete detachment from political institutions.

The Japanese Emperor’s role is more legalistic and symbolic, serving as a spiritual symbol of national unity and maintaining political neutrality. Culturally and traditionally, the Emperor holds a unique status, presiding over rituals such as the Enthronement Ceremony and the Niiname-sai (harvest festival), which preserve the religious and historical significance of the imperial household. In modern times, the Emperor continues to strengthen national cohesion and social identity by visiting disaster-stricken areas and participating in public events, “consistently conveying the message of being with the Constitution and the people, and always speaking in dialogue with the people.”[4] Unlike the British monarchy’s emphasis on historical continuity, the Japanese imperial system is firmly embedded in the post-war written constitution, embodying the principles of pacifism and the rule of law. It places greater emphasis on a symbolic and neutral role within the legal framework.

3. Differences in the Relationship Between Parliament and the Cabinet

In the United Kingdom, Parliament is supreme, and its core principle is the doctrine of “parliamentary sovereignty,” meaning that all state power originates from Parliament and is distributed by it. All branches of government are accountable to Parliament and subject to its oversight. The UK Parliament consists of the House of Lords and the House of Commons. The latter, as an elected body, holds substantive legislative power, while the former—composed mainly of hereditary and life peers—functions primarily in a reviewing and advisory capacity, with relatively limited authority. As the highest organ of state power, Parliament “enjoys comprehensive and exclusive legislative power, including the authority to enact, amend, and repeal any law”[5]. It also exercises oversight over the Cabinet, and all government bills, especially budget proposals, must be debated and voted on by Parliament. The House of Commons holds the power of a vote of no confidence, which can compel the Prime Minister and Cabinet to resign—an important mechanism for checking executive power. Additionally, Members of Parliament may question the Prime Minister and Cabinet ministers during “Prime Minister’s Questions” (PMQs), thereby reinforcing government accountability. The House of Commons also exercises absolute control over financial matters, as all tax policies and budget bills must receive parliamentary approval.

The UK Cabinet, as the core of executive power, consists of the Prime Minister and Cabinet ministers, who are responsible for “initiating and directing policy while being subject to parliamentary oversight and criticism”[6]. The Prime Minister, as head of government and leader of the majority party in Parliament, wields significant power—controlling the Cabinet’s agenda and exerting substantial influence over the direction of national governance. The Cabinet is collectively accountable to Parliament and must retain the confidence of the House of Commons. In the past, the Prime Minister could recommend that the monarch dissolve Parliament, but this power was limited by the Fixed-term Parliaments Act 2011, and then partially restored by the Dissolution and Calling of Parliament Act 2022. Although Parliament holds ultimate legislative authority, the Prime Minister usually commands a parliamentary majority, increasing the likelihood of government bills passing.

In general, the relationship between Parliament and the Cabinet in the UK can be characterized as a “strong Parliament–strong Prime Minister” model: Parliament exercises robust oversight over the government, while the Prime Minister also maintains considerable control over Parliament. Although Parliament can oust the government through a no-confidence vote at any time, the Prime Minister—typically leading the majority party—often enjoys sufficient support in Parliament, allowing the Cabinet to play a dominant role in policy-making.

By contrast, Japan’s National Diet occupies a comparatively weaker position. The Constitution of Japan establishes a parliamentary cabinet system. Although the Diet is the sole legislative body of the state, its ability to check executive power is not as strong as that of its British counterpart. The Japanese Diet consists of the House of Representatives and the House of Councillors. The former holds greater authority, particularly in matters such as budget approval and the nomination of the Prime Minister. When the two houses disagree, “the decision of the House of Representatives shall be the decision of the Diet”[7]. The House of Councillors serves mainly in a reviewing and supplementary capacity, with relatively limited power. The Diet is responsible for drafting and revising laws, and its enacted legislation holds the highest authority. It also supervises the Cabinet through interpellations and specialized committees to ensure the legality and effectiveness of executive actions. Moreover, the Diet controls the national budget, and all budget proposals must be deliberated and approved by the House of Representatives. The House of Representatives can also pass a vote of no confidence to force the Cabinet’s resignation or be dissolved by the Prime Minister for a new general election. While Diet members may question the Prime Minister and Cabinet ministers, “Prime Minister’s Questions” are not held weekly in Japan, making the oversight function relatively weaker.

Similar to the UK, the Japanese Prime Minister serves as head of the Cabinet and is nominated by the House of Representatives and appointed by the Emperor. However, unlike the UK Prime Minister, the Japanese Prime Minister “holds a position within the Cabinet more akin to that of a U.S. President”[8]—nominally in charge of the executive branch but in practice constrained by the Cabinet’s collective decision-making mechanism and the governing coalition. Cabinet decision-making requires intra-party consultation, and major policies must receive unanimous approval from all Cabinet members, making factional dynamics a significant influence. Although the Prime Minister holds the power to dissolve the House of Representatives, in practice this must be weighed against the prevailing political climate and internal party considerations. The Cabinet may submit bills to the Diet, and proposals from the ruling party typically pass when it holds a majority.

Overall, the relationship between Japan’s Diet and Cabinet can be described as a “weak parliament–relatively weak prime minister” model: the Diet exercises limited constraints over the Cabinet, and the Cabinet remains heavily dependent on parliamentary support. Although the Diet holds supervisory authority, accountability mechanisms are relatively weak—particularly given the limited powers of the House of Councillors, which struggles to counterbalance the House of Representatives. Additionally, the Prime Minister must navigate internal party factions, which weakens control over the Cabinet and policy agenda. As a result, the Japanese Cabinet relies heavily on securing a majority in the House of Representatives to effectively implement legislative and policy initiatives.

4. Differences in the Relationship Between Parliament, Cabinet, and the Judiciary

The United Kingdom adheres to the principle of “parliamentary sovereignty,” meaning that the judiciary does not possess constitutional review power and cannot overturn laws enacted by Parliament. Courts are “expected to interpret the law in accordance with precedent and reason”[9] to adapt to societal changes, but they cannot directly challenge the legislative authority of Parliament. The Human Rights Act grants courts the power to assess whether parliamentary legislation aligns with the European Convention on Human Rights; however, courts may only issue declarations of incompatibility and have no authority to invalidate legislation. The Cabinet must respect judicial independence and is prohibited from interfering with court rulings. Although the Prime Minister can indirectly influence the judiciary through the appointment of the Lord Chancellor and nomination of senior judges, such influence generally does not undermine judicial independence.

Judicial power in the UK is exercised independently. The Constitutional Reform Act 2005 established the Supreme Court, replacing the Appellate Committee of the House of Lords, thereby ensuring a complete institutional separation between the judiciary, Parliament, and Cabinet, and further reinforcing judicial independence. Judicial appointments are managed by the Judicial Appointments Commission, minimizing direct interference from the executive or legislative branches. As a common law system, the UK emphasizes the role of precedent, allowing courts considerable flexibility and creativity in legal interpretation and application. While courts do not possess the power to declare legislation unconstitutional, “they must not shrink from deciding whether the government has overstepped the mark or whether the boundary has become so blurred as to be meaningless”[10]. Nonetheless, courts may only assess the reasonableness and legality of government actions and may not directly negate administrative policies; thus, the scope of judicial review remains limited.

By contrast, Japanese courts hold greater authority. Article 76 of the Constitution of Japan stipulates that judicial power shall be exercised independently and that neither administrative agencies nor the legislature may interfere in judicial proceedings. The Supreme Court, as the highest judicial authority, possesses the power of constitutional interpretation and may review the constitutionality of laws, regulations, and administrative orders. Japanese courts have broad powers of constitutional review, allowing them to invalidate unconstitutional laws or administrative actions and to overturn legislation or executive decisions that conflict with the Constitution. As a result of the “strengthening of judicial power, government control increasingly depends on the courts and the system of constitutional review”[11]. The Supreme Court may conduct constitutionality reviews of electoral laws, fiscal legislation, and other statutes and holds ultimate authority in constitutional interpretation. Lower courts may also exercise constitutional review in specific cases, though in practice, such overturns are relatively infrequent.

Although Japanese courts enjoy broad powers of constitutional review, they are still subject to certain constraints from the legislature and executive. The Chief Justice of the Supreme Court is nominated by the Prime Minister and officially appointed by the Cabinet, which also appoints other justices. However, the independence of the judiciary is protected by the Constitution to prevent administrative interference. Moreover, Japanese judges must undergo periodic public reviews to ensure judicial fairness and transparency, although “this review applies only to Supreme Court justices and not to lower court judges”[12]. While the executive branch cannot interfere with judicial rulings, it may still influence the judicial system indirectly through budget allocation and personnel arrangements.

In summary, UK courts are limited by the principle of parliamentary sovereignty and may conduct only limited judicial review, whereas Japanese courts hold constitutional interpretive authority and can invalidate unconstitutional legislation or executive actions. However, in practice, Japan’s judiciary is still subject to influence from the legislative and executive branches, especially in areas such as judicial appointments and budgetary control. Each country’s judicial system thus reflects its unique political and legal structure.

5. Conclusion

The United Kingdom and Japan exhibit significant differences in their political systems, particularly in power structures and political culture. The UK’s system has evolved gradually over time, culminating in a parliamentary democracy centered on parliamentary sovereignty. The Cabinet is composed of members from the parliamentary majority, with the Prime Minister responsible for executive governance and accountable to Parliament. The judiciary plays a limited role, focusing on administrative review without the power to overturn parliamentary legislation. Japan, by contrast, underwent dramatic historical transformations, especially following World War II, with the postwar Constitution of Japan establishing a system of separation of powers. Under this framework, the Diet holds legislative authority, the Cabinet manages administrative affairs, and the judiciary is granted constitutional review powers, resulting in a relatively balanced distribution of power. Nevertheless, the ruling party still exercises significant control over both the Diet and Cabinet. The two countries also differ in terms of their symbolic heads of state. The UK monarchy has evolved into a purely symbolic institution, representing national unity and historical continuity. In Japan, the Emperor’s role has been strictly limited to a symbolic function since the postwar era, with all political power stripped away. The operational models of power and checks and balances also differ. In the UK, Parliament and the Cabinet maintain a close relationship, with executive power highly centralized and judicial review authority limited. In Japan, although the three branches of government are formally independent, the Diet can supervise the Cabinet through no-confidence resolutions, and the judiciary holds constitutional review powers. However, the dominance of the ruling party can lead to imbalances in practice. Thus, while the institutional designs suggest formal separations of power, political realities in each country shape the actual functioning of their systems.