The Sewel convention is named after Lord Sewel, who was the Scotland Office minister in the Lords responsible for the conduct of the Scotland bill in 1998. It provides that the UK Parliament may not legislate for devolved matters without the consent of the devolved legislature affected. Motions giving consent under the convention are now known as Legislative Consent Motions.
During the second reading debate, he said:
as happened in Northern Ireland earlier in the century, we would expect a convention to be established that Westminster would not normally legislate with regard to devolved matters in Scotland without the consent of the Scottish parliament. (HL Debates, volume no. 592, part no. 191, 21 July 1998, column 791.)
The Memorandum of Understanding (the main intergovernmental agreement)( sets out the convention more fully:
The United Kingdom Parliament retains authority to legislate on any issue, whether devolved or not. It is ultimately for Parliament to decide what use to make of that power. However, the UK Government will proceed in accordance with the convention that the UK Parliament would not normally legislate with regard to devolved matters except with the agreement of the devolved legislature. The devolved administrations will be responsible for seeking such agreement as may be required for this purpose on an approach from the UK Government. (para. 14)
The convention is treated as applying not just to substantive Westminster legislation on devolved matters like health and education, but also to constitutional changes which increase or reduce devolved executive or legislative powers. See for example Devolution Guidance Note 10 issued by the UK Ministry of Justice, on Post devolution Primary Legislation Affecting Scotland (available here). (Parallel guidance has been issued by the Scottish Government.) As regards Wales, DGN 9 on Post Devolution Primary Legislation Affecting Wales is less clear about this, but in any event will need revising following the result of the March 2011 referendum on the Assembly’s legislative power. (It’s available here.) This means that the powers of the Scottish Parliament and National Assembly for Wales to make laws can’t be reduced without their consent. It similarly means that the powers of the Scottish Government and Welsh Assembly Government can’t be reduced without the consent of the Parliament or National Assembly. The same applies to conferring new powers – extra powers cannot be compulsorily devolved, but only with devolved consent.
In principle, the convention applies equally to Northern Ireland, as DGN 8 on Post Devolution Primary Legislation Affecting Northern Ireland (available here) provides. However, very few legislative consent motions have ever been considered in the Northern Ireland Assembly, and in practice the power to give assent is usually exercised by the Northern Ireland Executive.
One significant shortcoming of the convention is that, although it is recognised by the governments involved, it has never been endorsed by the UK Parliament, even by a motion or resolution.
A House of Commons Library Briefing on the convention is available here. The Scotland Office’s account of it is set out here. That of the Scottish Government is here, and there’s information from the Scottish Government about its use here. The Scottish Parliament’s page about the working of legislative consent, with details of legislative consent memoranda submitted to it by the Scottish Government, is here. The National Assembly for Wales now tracks progress of LCMs here.