Sir Stephen Laws QC is a Senior Research Fellow at Policy Exchange and was First Parliamentary Counsel from 2006-12.

Could backbench MPs engineer a change to the law to postpone Brexit without the Government’s help? Many seem to think so, judging by amendments that have been discussed over the last few days.

In my view, this is not only a mistaken assumption, but could lead to some potentially horrific consequences for our constitution. It may even see the Queen getting dragged into a legislative showdown between Parliament and the Government. As I explain in my Policy Exchange Research Note today, the prospect of it this is unthinkably awful. But sadly, not beyond the realms of possibility.

I should be clear from the start: this isn’t about whether you were for Leave or Remain at the referendum, or support Theresa May’s deal, but how Parliament is supposed to function at any time. The main misconception here appears to be that there is a way for a majority in the Commons to change the law on what happens next – never mind the opposition of the Government. That is simply not the case. All the House can do is try to persuade the Government to initiate the required legal changes. If that fails, it can pass a motion of no confidence which could in due course lead to the removal of the Government from office. It recently decided not to do that.

The idea that there must be some other way for a majority in the Commons to get its way on this is just wrong, and beyond the ingenuity of even the most cunning proceduralists. Let me explain why. Only changes to the law can stop the progress of any changes that have already been set in law by an Act of Parliament. In this context, we are talking about the European Union (Withdrawal) Act 2018, which repeals the European Communities Act 1972 from “exit day” on 29th March 2019. The Government has a clear legal duty to ensure that the repeal comes into force on that day and that the UK does not remain a member of the EU.

There are only two ways in which the repeal can be stopped from happening on that day. The first is by an order under section 20(4) of the 2018 Act, which postpones that day. Only the Government has the power to make the order – and then only if it has agreed an extension of the Article 50 period with all the other members of the EU. The order then needs to be agreed by each House of Parliament. The other way is by means of new primary legislation, which would have to be passed by both Houses.

Some MPs think that the only obstacle to the passage of such legislation against the wishes of the Government is a particular rule – Standing Order 14 – that gives Government business precedence in the House of Commons. A draft has been published of an amendment that might be tacked on to some future Government “business of the House” motion to disapply SO 14. Reliance is apparently being placed, no doubt with justified confidence, on the Speaker’s “creative” interpretation of what is permissible for the purpose of obstructing Brexit. But even the amendment that has been published does not go so far as to allow a non-Government Bill to be passed. It only allows a non-Government motion to have precedence on a named day, if it has a defined level of support.

Another very significant point has been overlooked. The repeals that are to come into force under the 2018 Act on March 29th include the repeal of section 2(3) of the 1972 Act. That is the provision that provides for the UK’s financial obligations to the EU and means that they have to be paid without the need to be voted for by Parliament on an annual basis.

It follows that any legislation to produce a postponement of the repeal, or its cancellation, will revive that provision (if only temporarily) and so be changing the law in a way that potentially carries a very substantial financial burden on the exchequer. It may be that the expenditure would continue under the proposals for a transition period, but there is no legislation for that yet in place and it cannot be taken into account. Nor could a further referendum be held without significant further Government expenditure.

The constitutional position is clear on this. A Bill that contains provisions that give rise to expenditure cannot pass – unless they have been authorised by a resolution of the House, and the motion for such a resolution can only be moved if it has been recommended by the Crown.

That means that the Government has to have approved it. The rules of the House – Standing Orders 48 and 49, to be precise – are very clear about this and also require the Government to approve their own disapplication.

Surely, you may ask, would not the Speaker’s “creative” approach to the rules of the House allow MPs some wiggle-room? In theory, I suppose it could; but it would be more than surprising and potentially horrific if it did so in practice. Those Standing Orders are fundamental elements of the constitutional relationship between Government and Parliament. Not only are they more than 150 years old, they actually derive from much longer ago than that: in the origins of the Commons as a body asked to respond to requests from the Crown for money, and to grant the requests in return for the redress of grievances.

The electoral system itself, so far as it is a means of holding the Government accountable to the public, depends crucially on the ability of the public to hold the Government responsible for how it has used the principal lever of government – the use of public money. Removing the responsibility would undermine the whole UK constitutional system.

So suppose the Speaker did allow an attempt to bypass the financial Standing Orders and allowed a Bill to pass that contravened them, and so to proceed to the Lords and be passed there. What would happen when the Bill then fell to be submitted for Royal Assent?

The question would inevitably arise whether the Government could reassert its wrongly denied constitutional veto on such a Bill by advising the Monarch not to grant Royal Assent to the Bill? Would it even, perhaps, think that it actually had a duty to ensure that a Bill that had been passed in contravention of fundamental constitutional principles did not reach the statute book?

It is a sacred duty of all UK politicians not involve the Monarch in politics, but might a Government in that situation think that this was precisely the last resort for which the Royal Assent process is retained? How should the Monarch react to such advice? The answer is not straightforward and the prospect of it needing to be considered in a real-life political crisis is a constitutional scenario of nightmares.