European Union (Notification of Withdrawal) Act 2017 in the context of "Theresa May"

The impact of Brexit on the Irish border and its adjacent polities involves changes in trade, customs, immigration checks, local economies, services, recognition of qualifications, medical cooperation, and other matters, as it is the only land border between the United Kingdom and the European Union.

After the UK Parliament voted to leave the European Union, all parties said that they want to avoid a hard border in Ireland, due particularly to the border's historically sensitive nature. Border issues were one of three areas of focused negotiation in the Withdrawal Agreement. Following the United Kingdom's exit from the European Union on 31 January 2020, this border is also the frontier between the EU and an external country.

On 29 March 2017, the United Kingdom (UK) invoked Article 50 of the Treaty on European Union which began the member state's withdrawal, commonly known as Brexit, from the European Union (EU). In compliance with the treaty, the UK gave formal notice to the European Council of its intention to withdraw from the EU to allow withdrawal negotiations to begin.

The process of leaving the EU was begun after a referendum held in June 2016 at which 52% of the votes were in favour of UK withdrawal. In October 2016, the UK prime minister, Theresa May, announced that Article 50 would be invoked by "the first quarter of 2017". On 24 January 2017 the Supreme Court ruled in Miller I that the process could not be initiated without an authorising Act of Parliament, and unanimously ruled against the Scottish Government's claim in respect of devolution. Consequently, the European Union (Notification of Withdrawal) Act 2017 empowering the prime minister to invoke Article 50 was enacted in March 2017.

R (Miller) v Secretary of State for Exiting the European Union is a United Kingdom constitutional law case decided by the United Kingdom Supreme Court on 24 January 2017, which ruled that the British Government (the executive) could not initiate withdrawal from the European Union by formal notification to the Council of the European Union as prescribed by Article 50 of the Treaty on European Union without an Act of Parliament first being passed to give the government Parliament's permission to do so. Two days later, the government responded by bringing to Parliament the European Union (Notification of Withdrawal) Act 2017 for first reading in the House of Commons on 26 January 2017. The case is informally referred to as "the Miller case" or "Miller I" (to differentiate with Miller's later Brexit-related case against the Government, Miller II).

The Supreme Court's decision was given on appeal from the High Court's ruling that the Crown's foreign affairs prerogative, which is exercised by the government led by the Prime Minister, may not be used to nullify rights that Parliament has enacted through primary legislation. The case was seen as having constitutional significance in deciding the scope of the royal prerogative in foreign affairs. The Supreme Court also ruled that devolved legislatures in Scotland, Wales and Northern Ireland have no legal right to veto the act.