The laws that led to the setting of devolved parliaments and assemblies made clear that devolution did not affect the power of the UK parliament to make laws for Scotland, Wales or Northern Ireland. However, a constitutional convention established that the UK parliament would ‘not normally’ legislate in areas that are devolved without the agreement of the devolved institutions. That convention – more commonly known as the Sewel Convention – has become both an important principle of devolution and a common practice of government. Its importance was recognised by its inclusion in the most recent reforms to the devolution laws setting out the powers of the Scottish and Welsh parliaments.

In line with the Convention, any law being considered by the UK parliament that include the purpose of changing devolved laws or devolved powers is accompanied by a process to secure the agreement, or legislative consent, of the devolved institutions. In this case, a devolved government will set out the proposed changes, and its view of these changes, in a legislative consent memorandum. This is often considered by relevant parliamentary committees before the parliamentarians themselves are invited to give or withhold their consent for the change. A refusal to grant consent does not mean that the change will not be made – because the UK parliament is sovereign, it can make or unmake any laws as it sees fit, including those that have a devolved purpose. But, by withholding their consent, the devolved institutions can often push the UK government to make changes to such laws so that the Sewel convention can be upheld.

The Sewel convention has been widely respected and applied by successive UK governments and parliaments since 1999. It has, however, been tested by the Brexit process.

In 2018, the European Union (Withdrawal) Act, paving the way for the UK’s EU exit, was passed by the UK parliament despite the withholding of consent by the Scottish Parliament, whose members opposed the changes it made to devolved powers. In January 2020, the European Union (Withdrawal Agreement) Act, putting the exit deal into UK law, was passed by the UK parliament without the consent of any of the devolved institutions. This marked the first time that the Scottish Parliament, the National Assembly for Wales and the Northern Ireland Assembly have together refused consent for a UK bill. The fact that it made no difference to the passage of the law has exposed the limitations of a convention with no legal effect.