Judicial functions of the House of Lords in the context of "King-in-Parliament"

The constitution of the United Kingdom comprises the written and unwritten arrangements that establish the United Kingdom of Great Britain and Northern Ireland as a political body. Unlike in most countries, no official attempt has been made to codify such arrangements into a single document, thus it is known as an uncodified constitution. This enables the constitution to be easily changed as no provisions are formally entrenched.

The Supreme Court of the United Kingdom and its predecessor, the Appellate Committee of the House of Lords, have recognised and affirmed constitutional principles such as parliamentary sovereignty, the rule of law, democracy, and upholding international law. It also recognises that some Acts of Parliament have special constitutional status. These include the Magna Carta, which in 1215 required the King to call a "common counsel" (now called Parliament) to represent the people, to hold courts in a fixed place, to guarantee fair trials, to guarantee free movement of people, to free the church from the state, and to guarantee rights of "common" people to use the land. After the Glorious Revolution, the Bill of Rights 1689 and the Claim of Right Act 1689 cemented Parliament's position as the supreme law-making body, and said that the "election of members of Parliament ought to be free". The Treaty of Union in 1706 and the Acts of Union 1707 united the Kingdoms of England, Wales and Scotland, the Acts of Union 1800 joined Ireland, but the Irish Free State separated after the Anglo-Irish Treaty in 1922, leaving Northern Ireland within the UK. After struggles for universal suffrage, the UK guaranteed every adult citizen over 21 years the equal right to vote in the Representation of the People (Equal Franchise) Act 1928. After World War II, the UK became a founding member of the Council of Europe to uphold human rights, and the United Nations to guarantee international peace and security. The UK was a member of the European Union, joining its predecessor in 1973, but left in 2020. The UK is also a founding member of the International Labour Organization and the World Trade Organization to participate in regulating the global economy.

The Appellate Jurisdiction Act 1876 (39 & 40 Vict. c. 59) was an act of the Parliament of the United Kingdom that altered the judicial functions of the House of Lords by allowing senior judges to sit in the House of Lords as life peers with the rank of baron, known as Lords of Appeal in Ordinary. The first person to be made a law lord under its terms was Sir Colin Blackburn on 16 October 1876, who became Baron Blackburn.

The president of the Supreme Court of the United Kingdom is the highest-ranking judge in the Supreme Court of the United Kingdom. It is equivalent to the now-defunct position of Senior Lord of Appeal in Ordinary, also known as the Senior Law Lord, who was the highest ranking among the Lords of Appeal in Ordinary (the judges who exercised the judicial functions of the House of Lords). The current President is Robert Reed, since 13 January 2020.