Partner Charles Livingstone gave a live interview on BBC News on 13 March 2017.

"Section 30" refers to Section 30 of the Scotland Act 1998.

This empowers the Queen (though in truth the UK Government) to make an Order in Council amending the list of matters that are reserved to the UK Parliament, which appear in Schedule 5 of the Act.

Section 30 Orders can only be made if a draft is first placed before the Scottish Parliament, House of Commons and House of Lords, and approved by all three.

These legal constraints essentially give each institution a veto over the terms of an order.

One of the reserved matters is "the Union of the Kingdoms of Scotland and England".

One of the opening arguments of the 2014 referendum was whether this meant the Scottish Parliament did not have the power to legislate for a referendum.

At the time the Scottish Government argued that it did have that power as any referendum would not be binding - it would merely be a testing of public opinion on the question of independence.

The question was never conclusively answered because a Section 30 Order was made, which introduced an exception to Schedule 5 that allowed for a one-question referendum to take place before the end of 2014.

While the Scottish Government has never expressly changed its view on that question, it does now seem to be proceeding as if a Section 30 Order is necessary.

However, that may be tested if an Order is refused by the UK Government, or offered subject to qualifications that the Scottish Government deems unacceptable.

In that case the Scottish Government could seek to introduce its draft legislation without an order.

The Presiding Officer of the Scottish Parliament has to make a statement on whether any Bill introduced to the Scottish Parliament is within legislative competence, but even if he decides it is not that does not bar the Bill from proceeding.

If the Bill was passed there would be a four-week 'standstill' period prior to Royal Assent, in which one of the UK Government Law Officers (the Advocate General or Attorney General) could ask the UK Supreme Court to decide whether it was within competence.

Otherwise the legislation could be challenged in court by any party who could show a sufficient interest.

The court would then decide whether the legislation was lawful, and so whether the referendum could go ahead.

Charles Livingstone is a Partner in the Public Law and Regulatory team at Brodies LLP.