This is an updated version of a blog post previously published on 27 June 2016
During the debate in the Westminster Parliament following the Queen’s Speech yesterday (21 June 2017) Theresa May suggested that a legislative consent motion may be required in the Scottish Parliament during the passage of the Repeal Bill, one of the 8 bills proposed in the Queen’s Speech to implement the UK’s withdrawal from the EU (see here, at column 62).
It is a well established constitutional convention, known as the Sewel Convention, that the UK Government will not normally legislate on devolved matters except with the agreement of the devolved legislature (see paragraph 14 of the Memorandum of Understanding between the UK Government and devolved executives (October 2013)). In Scotland, the method of obtaining this agreement is through a legislative consent motion passed by the Scottish Parliament. Since its inception in 1998, the Sewel Convention has evolved to require consent “not only where the UK Parliament seeks to legislate in devolved policy areas, but beyond that where a UK bill seeks to vary the legislative competence of the Scottish Parliament or the executive competence of the Scottish Ministers.” (Chris McCorkindale, Scottish Constitutional Futures Forum). This expansion of the convention has been recognised in practice and is reflected in Rule 9B.1 of the Standing Orders of the Scottish Parliament.
Leaving the European Union is likely to increase the legislative competence of the Scottish Parliament as on Brexit day areas previously governed by EU law (such as fisheries and agriculture) would, without further enactment, become devolved issues within the competence of the Scottish Parliament. This is because anything that is not expressly reserved is devolved. The restriction in section 29(2)(d) of the Scotland Act 1998 preventing the Scottish Parliament from passing an Act which is incompatible with EU law would also become meaningless, which could also be said to increase the Parliament’s competence.
This would be the default position. However, there is also the possibility that the Repeal Bill will seek to amend the Scotland Act 1998 to ensure that certain devolved issues currently dealt with at the EU level will be repatriated to Westminster rather than Holyrood. Westminster could then decide what to devolve to Holyrood and what to retain at Westminster. As highlighted by Mark Elliot, this appears to be the approach suggested in the briefing notes which accompanied the Queens Speech (see here). This would certainly require the legislative consent of the Scottish Parliament as it would require amendments to the Scotland Act 1998.
However, Brexit will not be “blocked” if the Scottish Parliament refuses consent. The Sewel Convention is a purely political convention: it is not legally enforceable. This was recently confirmed in Miller v Secretary of State for Exiting the European Union [2017 UKSC 5 where the Supreme Court held that judges:
“are neither the parents nor the guardians of political conventions; they are merely observers. As such, they can recognise the operation of a political convention in the context of deciding a legal question... but they cannot give legal rulings on its operation or scope, because those matters are determined within the political world.” 
At paragraph 150, the court agreed with submissions from the Lord Advocate (who represented the Scottish Government) and the Counsel General for Wales that the Scottish Parliament and the Welsh Assembly do not have a legal veto on the UK’s withdrawal from the EU.
The Scottish Parliament could certainly cause a lot of political commotion and controversy by refusing consent, however the refusal would have no effect on Westminster’s ability to pass the Repeal Bill into law. It is therefore misleading to suggest that the Scottish Parliament can block or veto the UK’s withdrawal from the EU.