One of the ironies of the government's agonies over the 'meaningful vote' is that it did not have to hold it at all.

In our work on Parliament's role in treaty-making, we distinguish between four potential decision-making stages in the process: opening negotiations; signature; the passing of legislation where necessary; and ratification.

In the default process by which the UK makes treaties (when it is acting alone rather than as part of the EU), Parliament has no established role in the first two stages. Our research shows that, for the majority of treaties, Parliament then does need to legislate in some way. And then, for treaties that require ratification, under the Constitutional Reform and Governance Act 2010 (the CRaG Act) the House of Commons has only a negative consent power - that is, the government may ratify unless the House objects; there is no requirement for a positive vote.

If the Withdrawal Agreement had been handled according to the default process, therefore, Parliament could in theory have been uninvolved until it was required to consider the Bill needed for implementation. Such legislation is not introduced until after treaties are signed. And passing that Bill would have generated what could have been the only parliamentary votes in the whole process.

Instead, as the result of a process-about-the-process that has lasted over two years and involved a court case, two Acts of Parliament and an amendment to a House of Commons business motion, the Withdrawal Agreement may prove to be the UK treaty with the most parliamentary decision-making involvement ever.

The Withdrawal Agreement process thus throws into relief the nature of the default arrangements for Parliament's role in treaty-making.

But, in doing so, it also highlights the flexibility that is available within the UK system to vary the role of Parliament in the treaty-making process. To a significant extent, a 'pick n' mix' is available, of different potential elements to put together to make up the process for parliamentary engagement on any one treaty.

This flexibility and scope for choice matters because the shape that Parliament's role in treaty-making should take in future is currently under active consideration, with the House of Lords Constitution Committee and - with respect to trade agreements - the House of Commons International Trade Committee both conducting inquiries into the question.

And, of course, even the final Withdrawal Agreement process may yet get even more unusual: the debate over the 'People's Vote' is in effect a debate over whether the UK should join the ranks of countries that sometimes put treaty approvals to referendums (a possibility that was raised by the referendum requirements of the European Union Act 2011 but never realised).

So, for those interested in playing 'pick n'mix' with the mechanics of parliamentary engagement in treaty-making, here are:

1. The 'meaningful vote' comes towards the end of a treaty-making process which required parliamentary authorisation to launch.

As a result of the Miller case, the government could not open the negotiations for the Withdrawal Agreement (by triggering Article 50 of the Treaty on European Union, TEU) without parliamentary authorisation. This took the form of the European Union (Notification of Withdrawal) Act 2017.

Normally, as noted above, Parliament has no role at this stage of the process.

2. In the 'meaningful vote', Parliament is being asked to approve the Withdrawal Agreement before it is signed.

We know of only one other case where this occurred, among treaties made by the UK acting alone rather than as part of the EU: the 1984 Sino-British Joint Declaration on Hong Kong.

In practice, as MPs are being reminded with respect to the Withdrawal Agreement, it is hard to affect the content of a treaty any time after political agreement on it has been reached, whether or not it has been signed.

But these difficulties are compounded once a treaty is signed; and the act of signature brings some obligations under international law, even if a state is yet to give its final 'consent to be bound' through ratification.

3. There is a statutory requirement that the House of Commons must approve the Withdrawal Agreement by resolution - the 'meaningful vote' - as a condition for ratification.

This is established by section 13 of the EU (Withdrawal) Act 2018.

We know of no other case in which this applied.

This feature of the 'meaningful vote' process rests on some procedural and legal distinctions:

a) There have been cases where there was a statutory requirement for Parliament to approve treaties as a condition for ratification. However, in these cases, the law required that parliamentary approval take the form of an Act of Parliament, rather than a resolution.

The cases where this applied were new EU Treaties which expanded the competences or objectives of the EU or the powers of its institutions. Successive EEC/EC/EU-related Acts required passage of a further Act as a condition for UK ratification of such Treaties.

b) There have also been cases in which Parliament has approved a treaty by resolution prior to ratification or its otherwise coming into effect. However, in these cases, this was not a statutory requirement.

Cases we know of such as this, where both Houses approved treaties by resolution, are the UN Charter (1945), the North Atlantic Treaty (1949) and the 1985 Anglo-Irish Agreement.

4. The Withdrawal Agreement is to be subject to all three of: an approval motion; legislation; and negative parliamentary consent to ratification.

The only two other cases we know of where this applied were the Sino-British Joint Declaration on Hong Kong, again; and the UN Charter.

More commonly, treaties are subject to only one or two of the three elements. The different forms of potential parliamentary engagement - and the different strength of the roles that they afford Parliament - are seen as balancing each other out, and sometimes negating the need entirely for one or another of the other possible parliamentary actions.

For example:

• Approval motions have sometimes been used to 'compensate' in cases where treaties would not require legislation, as in the case of the North Atlantic Treaty.

• The most common pattern for parliamentary engagement in treaty-making is legislation + negative consent to ratification, and the traditional justification for Parliament's weak powers over ratification is that it has a strong role through legislation.

• Part of the rationale for Parliament having any potential role in ratification, as set out by its author Arthur Ponsonby in 1924, was as an 'insurance' at the end of the process for treaties not requiring legislation.

• In cases where legislation is definitely required, however, the relevant treaties - EU Treaties and tax treaties - are exempted from the requirement for parliamentary consent to ratification under the CRaG Act.

This 'balancing-out' approach seems to underlie much parliamentary discussion of treaty-making. The approach seems to see the process almost as a unitary one, with the main concern being to ensure that Parliament at least gets a role 'somewhere', and it risks underplaying the significance of the point in the treaty-making process at which Parliament gets engaged.

5. For the Withdrawal Agreement, Parliament must complete the proceedings required for UK ratification by a specific date (29 March 2019), otherwise the treaty falls.

The only arguably similar case we have encountered is, yet again, the Sino-British Joint Declaration, which specified that the exchange of instruments of ratification "shall take place" before 30 June 1985, when the treaty would also come into force.

More commonly, treaties include a coming-into-force date which is not a named date but is specified instead in relation to the date on which (a certain number of) instruments of ratification are exchanged or deposited.

As a result, there is often no particular urgency about any one country's ratification process.

Indeed, our research suggests that this might be one of the problems surrounding current arrangements for parliamentary involvement in treaty-making. The 21-day pre-ratification parliamentary scrutiny period under the CRaG Act is relatively short, which potentially helps to discourage parliamentarians from engaging. However, when the scrutiny period expires, the lack of urgency surrounding the international ratification process can mean that nothing immediately happens, so parliamentarians risk getting engaged to no obvious immediate effect, or to authorise a ratification that could take place in future when conditions have changed.

6. The 'meaningful vote' is a treaty approval vote that is explicitly not just about the current treaty.

Under section 13(1)(b) of the EU (Withdrawal) Act, the 'meaningful vote' is required to approve simultaneously the Withdrawal Agreement and the framework for the future UK-EU relationship, now in the form of a Political Declaration.

We know of no other case where a parliamentary treaty approval action is explicitly not just about the treaty in question.

As the House of Commons Library pointed out at the time, the yoking together of the Withdrawal Agreement and the framework for the future relationship was, like the decision to have a 'meaningful vote' at all, a government one. The Hailsham amendment passed by the House of Lords to the then EU (Withdrawal) Bill, which is the textual origin of the section 13 eventually enacted, required that the House of Commons approve only the Withdrawal Agreement. The dual-purposing of the 'meaningful vote', to include the framework for the future relationship, was flagged by the government in December 2017 and duly appeared in June 2018 in the government's proposed amendment in lieu of the Hailsham text.

The government might have wished to see a conjoined approval vote on the Withdrawal Agreement and the framework for the future relationship simply to reflect the fact that, under the terms of Article 50 TEU, the two come as a package.

But the government's move might also have reflected one of two contradictory views of the framework for the future relationship:

• On the one hand, the framework might have been seen as a sweetener for the bitterer pill of the Withdrawal Agreement. This approach might make sense politically, but did not necessarily require the framework to be subject to a parliamentary approval vote.

• Alternatively, the framework might have been seen as sufficiently weak or controversial that, in order for it to win support, it needed to be packaged together with the Withdrawal Agreement and thus also made subject to the threat of 'no-deal'. However, again, in this case why subject the framework to parliamentary approval at all?

One possibility is that the government wanted to secure parliamentary approval for the framework because it sees the document as a form of mandate for its position in the forthcoming negotiations with the EU on the long-term UK-EU relationship.

Indeed, in opening the 'meaningful vote' debate on 4 December, the Prime Minister told the House of Commons that the political declaration set "the outline of (the) mandate" for the UK’s negotiation of the long-term UK-EU deal. She said:

for the next stage of negotiations we will ensure a greater and more formal role for Parliament. This will begin immediately as we develop our negotiating mandate, building on the political declaration ahead of 29 March 2019. The Government will consult more widely and engage more intensively with Parliament as we finalise the mandate for the next phase of the negotiations. … Members across the House will be able to contribute their expertise to the detailed positions we take forward with the EU, and the whole House will be consulted on the final version of that full mandate.

As the Withdrawal Agreement process draws towards its final stages, therefore, it could be followed by further innovation around Parliament’s role at the start of the treaty-making cycle.

1. Ivan A. Shearer, 'Non-Extradition of Nationals: A Review and a Proposal', Adelaide Law Review, Vol. 2 No. 3, 1966, p. 277; The Lord Templeman, 'Treaty-Making and the British Parliament', Chicago-Kent Law Review, Vol. 67 No. 2, 1991, p. 464

This post is an edited version of remarks made at the Constitution Society Winter Conference, Westminster, 30 November 2018. Thanks are due to the Constitution Society for the opportunity.

Any readers knowing of other unusual UK parliamentary treaty processes are encouraged to be in touch!