This is the Bill which when it becomes law will require the UK Government to trigger Article 50.
Note that the Bill imposes a duty on the Government, says what must be done and sets a deadline.
WITHDRAWAL FROM THE EUROPEAN UNION (ARTICLE 50) BILL (HC BILL 104)
A BILL TO Require Her Majesty’s Government to notify the European Council by 31 March 2017 of the United Kingdom’s intention to withdraw from the European Union.
Be it enacted by the Queen’s most Excellent Majesty, by and with the advice and consent of the Lords Spiritual and Temporal, and Commons, in this present Parliament assembled, and by the authority of the same, as follows:—
Section 1 Notification of the United Kingdom’s intention to withdraw from the European Union
(1) Her Majesty’s Government has a duty to notify the European Council by 31 March 2017 of the United Kingdom’s intention to withdraw from the European Union.
(2) The notification must meet the terms of Article 50 of the Treaty on European Union.
Section 2 Short title, commencement and extent
(1) This Act may be cited as the Withdrawal from the European Union (Article 50) Act 2017.
(2) The Act shall come into force on the day on which it receives Royal Assent.
(3) This Act extends to England and Wales, Scotland, and Northern Ireland.