R (Miller) v Secretary of State for Exiting the European Union  UKSC 5, sometimes referred to as the Miller I case, was an important constitutional judicial review regarding whether the government had the power to invoke Article 50 of the Treaty on European Union without Parliament’s approval.
In 2016, the British public had voted in a referendum to leave the European Union. The government’s intention to invoke Article 50, which would begin the formal process of leaving the EU, was challenged in court by a number of parties.
The claimants’ argument was that due to parliamentary sovereignty, the government did not have the power to override an Act of Parliament using the royal prerogative. The specific Act in this case was the European Communities Act 1972, which brought the UK into the European Economic Community (precursor for the EU).
The case was initially heard by the High Court, who judged in favour of the claimants, ruling that the government’s intention to use the royal prerogative was unlawful. The government appealed to the Supreme Court who denied the appeal and upheld the High Court’s decision. Shortly afterwards, the government introduced the European Union (Notification of Withdrawal) Bill to Parliament, which passed both Houses and received royal assent in March 2017.
The European Communities Act 1972 was eventually repealed by the European Union (Withdrawal) Act 2018.