The House of Commons library has published a paper clarifying the role of domestic law in extending Article 50.
Section 20(4) of the European Union (Withdrawal) Act 2018 allows a Minister of the Crown to change exit day provided that a draft statutory instrument has been laid before and approved by both the House of Commons and the House of Lords. However, it can only be used “to ensure that the day and time specified in the definition are the day and time that the Treaties are to cease to apply to the United Kingdom.” In practice this means that an extension must first have been agreed to at EU level before any such regulations can be made to change the date.
A Government written statement of Friday 15 March said:
“It is expected that the EU will use the March European Council on the 21 and 22 March 2019 to consider and reach a decision on a request from the UK to extend the Article 50 period. As soon as possible following agreement at the EU level we will bring forward the necessary legislation to amend the definition of exit day in domestic legislation. This statutory instrument will be laid, before it is made, under section 20(4) of the EU (Withdrawal) Act 2018. This legislation is subject to the draft affirmative procedure and so would need to be actively approved in each House. The legislation would give effect to any agreement with the EU on an extension, so would not be laid before Parliament until that agreement had been reached.”