Separation of powers in the United Kingdom

• Parliament - legislature
• Prime Minister, Cabinet, Government Departments & Civil Service - executive
• Courts - judiciary

Although the doctrine of separation of power plays a role in the United Kingdom's constitutional doctrine, the UK constitution is often described as having "a weak separation of powers" A. V. Dicey, despite its constitution being the one to which Montesquieu originally referred. For example, in the United Kingdom, the executive forms a subset of the legislature, as did—to a lesser extent—the judiciary until the establishment of the Supreme Court of the United Kingdom. The Prime Minister, the Chief Executive, sits as a member of the Parliament of the United Kingdom, either as a peer in the House of Lords or as an elected member of the House of Commons (by convention, and as a result of the supremacy of the Lower House, the Prime Minister now sits in the House of Commons) and can effectively be removed from office by a simple majority vote. Furthermore, while the courts in the United Kingdom are undoubtedly amongst the most independent in the world, the Law Lords, who were the final arbiters of judicial disputes in the UK sat simultaneously in the House of Lords, the upper house of the legislature, although this arrangement ceased in 2009 when the Supreme Court of the United Kingdom came into existence. Furthermore, because of the existence of Parliamentary sovereignty, while the theory of separation of powers may be studied there, a system such as that of the UK is more accurately described as a "fusion of powers".^{[citation needed]}

The development of the British constitution, which is not a codified document, is based on this fusion in the person of the Monarch, who has a formal role to play in the legislature (Parliament, which is where legal and political sovereignty lies, is the Crown-in-Parliament, and is summoned and dissolved by the Sovereign who must give his or her Royal Assent to all Bills so that they become Acts), the executive (the Sovereign appoints all ministers of His/Her Majesty's Government, who govern in the name of the Crown) and the judiciary (the Sovereign, as the fount of justice, appoints all senior judges, and all public prosecutions are brought in his or her name).

The British legal systems are based on common law traditions, which require:

• Police or regulators cannot initiate complaints under criminal law but can only investigate (prosecution is mostly reserved for the Crown Prosecution Service), which prevents selective enforcement, e.g. the 'fishing expedition' which is often specifically forbidden.
• Prosecutors cannot withhold evidence from counsel for the defendant; to do so results in mistrial or dismissal. Accordingly, their relation to police is no advantage.
• Defendants convicted can appeal, but only fresh and compelling evidence not available at trial can be introduced, restricting the power of the court of appeal to the process of law applied.