A treaty is an agreement between states, concluded in writing, which creates rights and obligations in international law. Treaty-making in the UK is predominantly an executive function, with only limited opportunity for scrutiny afforded to Parliament between agreement and ratification. Whilst Parliament can exert political pressure during the negotiation and agreement stages, the lack of any formal parliamentary involvement in treaty agreement has long prompted concerns about governmental accountability. This blog post explores the process of treaty making in the UK and evaluates the mechanisms for parliamentary scrutiny.
The process of treaty making in the UK
In the UK, treaty-making is an executive function, with the power to conclude a treaty deriving from the royal prerogative. The responsibility for treaty-making lies with the Secretary of State for Foreign, Commonwealth and Development Affairs, even when the negotiation of the treaty itself is led by an alternative government department.
Whilst Parliament does not have a systematic scrutiny function during the negotiation and agreement of a treaty, an agreed treaty must be laid before it for 21 sitting days prior to ratification. This rule, whilst traditionally operating by way of convention (known as the ‘Ponsonby rule’), was codified in Part 2 of the Constitutional Reform and Governance Act 2010. The 2010 Act does not, however, apply to all treaties. In exceptional cases, a government has the power under section 22 of the 2010 Act to exclude the process of scrutiny for a specific treaty. Moreover, other types of international arrangements, such as memoranda of understanding, are outside its scope.
Where the 2010 Act does apply and Parliament is afforded its 21 sitting-day window, either House may resolve that a treaty should not be ratified within that time. Whilst a resolution of the House of Lords may be overridden by a ministerial statement, repeated resolutions of the House of Commons may indefinitely delay ratification. This power of indefinite delay has not, however, been exercised by the House of Commons, nor has the House ever passed a resolution against the ratification of a treaty under the 2010 Act.[1] The House of Lords has passed a resolution against the ratification of a treaty only once, when in January 2024 it opposed the ratification of the UK-Rwanda Treaty. Despite this, in April 2024 the UK-Rwanda Treaty completed the ratification process, on the same day that the ministerial statement which overrode the resolution was made.
Because international treaties do not automatically become part of the domestic law of the UK, Parliament may need to pass implementing legislation to render a treaty effective. In particular, where the ratification of, or withdrawal from, a treaty will involve changes to domestic legislation, Parliament’s cooperation will be needed because domestic statutory law cannot be altered through the exercise of prerogative powers alone.[2] Given that this in effect introduces a secondary hurdle for a government in some circumstances, the Foreign Office guidance on treaties provides that where legislation is necessary, it ought to be in place prior to the ratification of that treaty so that the two can come into operation at the same time.[3] It is important to note, however, that domestic legislation will not always be needed; delegated powers to make secondary legislation (exercised by the executive) may be used instead, or only narrow sections of treaty agreements may be presented to Parliament to consider where the remaining obligations under a treaty can be met without legislating.
Do the present rules provide sufficient opportunity for Parliamentary scrutiny?
Much has been written about the inadequacy of the present rules and there are several respects in which greater Parliamentary involvement would be desirable.
As with any purely executive action, questions about democratic legitimacy can be raised where Parliament has no input into the substantive content of a treaty. In many cases, treaties are negotiated only by the executive, and often without transparency.[4] Even where, in some circumstances, Parliament will be responsible for any domestic legislation which implements treaty obligations, this is distinctive from parliamentary scrutiny of the treaty itself and simply involves the drafting of implementing legislation to facilitate the policy ends. As a recent House of Lords International Agreements Committee report identifies, “scrutiny of implementing legislation is no substitute for treaty scrutiny. Parliament should have the opportunity to consider the treaty as a whole so it can consider the policy reasons for the UK becoming a party.”[5] Allowing MPs an active role in contributing to the content and development of treaties facilitates greater democratic participation, and can provide longer-term political coherence across governments.
Moreover, the 21 sitting day window has also been criticised as too brief for significant treaties where effective parliamentary scrutiny is required.[6] The International Agreements Committee suggests that the need for MPs to obtain evidence about the treaty from stakeholders and subject-matter experts, and that the scrutiny period needs to accommodate both a committee report and a debate with sufficient time in between to allow MPs to consider the issues, render the window too short. Executive control of the parliamentary timetable also has the potential to render the short window obsolete. On two recent occasions, the last government refused requests made by the House of Commons to have a treaty debate within the 21 day period. The first was in January 2024, where the House of Commons Home Affairs Committee called for time to debate the Rwanda Asylum Partnership Agreement. The second was in February 2024, when the Business and Trade Committee called for a debate within the CRAG period of the treaty on accession of the UK to the Comprehensive and Progressive Agreement for a Trans-Pacific Partnership. Both requests were rejected on the basis that a parliamentary debate on the implementing legislation would be sufficient.[7] These examples demonstrate that even under the existing rules, the executive has the latitude to evade parliamentary, undermining the operation of the current accountability framework.
Other commentators, including the Hansard Society, have noted that the 2010 Act was passed in a different constitutional landscape. The Hansard Society suggests that, in a post-Brexit UK, the “number, range, complexity and impact of international agreements has grown significantly”. As such, it argues that the current legislative framework for parliamentary scrutiny of treaties is perhaps ill-suited to our new reality.
Whilst it is evident that Parliament could have a greater role to play in the scrutiny of treaty formation, the question of whether Parliament should have a greater role is perhaps more delicate. The balance of powers between the executive and the legislature sits at the heart of any debate for the reform of Parliament’s role. Any shift towards greater parliamentary control of treaty agreement would signal a dilution of executive freedom to negotiate with other states. Further, the ability to conclude treaties has long been viewed as a distinctly executive process, and one which may warrant special treatment when determining the level of scrutiny that might properly be applied by the judiciary and the legislature.[8] This does not, however, mean that treaties should be without any meaningful scrutiny from the other organs of the state, and at present, it appears that the scope for Parliament to scrutinise treaties is drawn too narrowly.
There does seem to be an increasing appetite for reform of the process, though. As the House of Commons Research Briefing ‘Treaty-making and Parliamentary scrutiny: recent developments’ notes:
All the Select Committees involved in treaty scrutiny during the 2019-24 Parliament agreed on the need for reform of existing arrangements. A recurring theme was Parliament’s inability to ensure that treaties could be debated before ratification and a sense that this goal was only likely to be achieved in practice if Parliament’s formal approval of a treaty was required.
Given the shared appetite for change across parties in the Commons and the Lords on this issue, movement does seem possible. A considered strengthening of the present accountability framework would be welcome.
Cassandra Somers-Joce.
Cassandra Somers-Joce studied law at Oxford University, where she is currently a non-stipendiary lecturer. She has previously taught public law at King’s College London, in addition to holding research roles at the University of York and Durham University.
The Constitution Society is committed to the promotion of informed debate and is politically impartial. Any views expressed in this article are the personal views of the author and not those of The Constitution Society.
[1] HL Paper 168, 11 September 2025, page 11.
[2] See the decision of the Supreme Court in R (Miller) v The Secretary of State for Exiting the European Union [2017] UKSC 5, and Attorney General v De Keyser’s Royal Hotel Ltd [1920] AC 508, per Lord Dunedin at page 526.
[3] The relevant paragraph of the guidance is paragraph 11, which provides that “Accordingly, if domestic legislation is required to enable the UK to give effect to its obligations under a treaty, the legislation should be in place before the treaty comes into force, so that the two can come into operation at the same time.” (emphasis original).
[4] HL Paper 168, 11 September 2025, page 7.
[5] HL Paper 168, 11 September 2025, page 18.
[6] Ibid, page 35.
[7] Ibid, page 17.
[8] This question has arisen in the context, for instance, of the justiciability of prerogative powers, in cases such as Council of Civil Service Unions v Minister for the Civil Service [1985] AC 374.