How can backbenchers take back control of Brexit? The latest plan is by Dominic Grieve who would (according to leaks) amend it to the Prime Minister’s new mystery Brexit plan which is being put to a vote on 29 January. As you might expect from Grieve, a QC, it’s well put-together. It identifies a weak point in Britain’s constitutional architecture, and proposes to take a shot. If he hits his target, it might not just take down Brexit but a whole lot more besides.

His amendment does not advocate for a particular policy, only against no deal. The Commons gave parliament permission for no-deal when it endorsed Article 50, so Grieve seeks to rescind that permission. He also aims to secure additional power for use in the future. In this case: control over time in the Commons.

For centuries under the Westminster system, power has rested in majorities. Only a government with a majority can govern, and government means ability to control the parliamentary timetable. The majority principle has always been crucial: a check against excessive government power, but also against wrecking intentions of a minority. As Bagehot put it: “The majority of the legislature, being well disposed to the Government, would not “find” against it except it had really committed some big and plain mistake.”

Grieve’s amendment would overturn this basic majoritarian principle. He would allow rebels to take control of the parliamentary timetable with a minority: 300 of the 650 MPs, or 46 per cent. If a motion reaches this number – and is backed by ten Tories and representatives from five parties - then it would be the first item of business the next day in the Commons. The numbers feel rather arbitrary. Also, a five party threshold elevates Caroline Lucas, as the single Green, to a power-broker, along with just 3pc of the governing party.

Grieve envisages that if that threshold was crossed, the proposal should be bounced straight on to the order paper, pushing aside whatever the government has planned, even if a majority of MPs oppose such an action. And we know already that the plan is not just to facilitate a debate, but to facilitate the introduction of legislation to overrturn the EU Withdrawal Act.

It appears designed to maximise the chance of being passed, while concealing the fact that there still is not a majority among remainers and anti-no-dealers on what should happen instead. It kicks the can down the road, waiting for a time when perhaps a second referendum or some other option might gather sufficient support.

There are two views on this. Some argue that there is extensive support for something to be debated, particularly on a matter of Brexit, then the government shouldn’t be able to block it. That we need not worry about the 46pc figure: an actual majority would be needed to pass the amendment, a majority would be needed to approve whatever motion was put before the House, that there are sufficient checks on this dramatic reduction in the government’s control. And those that back a House Business Committee to organise all business see it as perfectly permissible.

Others, like me, argue that this fundamentally undermines the executive and creates a worrying precedent. It threatens to unravel our system of government. Grieve’s motion would mean that even if a proposal was opposed by a majority in parliament if it could reach the stated threshold of 300 MPs, it would still have time on the floor of the House – power would pass to a minority. At the extreme end, it paves the way for future amendments to specify that if there is support from a particular cohort, or a particular number of MPs, even in the absence of a majority, that that will be sufficient for a matter to proceed.

We might scoff and say: who cares what MPs debate? No majority still means no majority. And a majority will be needed for anything serious. But there are no checks to ensure that this will just be a one-off - nothing to prevent a majority empowering a minority to do something else at a later date, or changing the rules to require a super-majority which the government can never overcome, only MPs themselves and the Speaker, and the rules are being torn up.

Rushed reforms are rarely good in the longer term. Even if Grieve were right in his aims, the speed at which he’s attempting to rip up convention and standing orders is dangerous.

All this will have far greater impact on how we are governed than, for example, House of Lords reform. Yet is being done on a whim, and by MPs who are in a fury. This may be regretted not just by parliament but the country - I can see it being the start of calls for a codified Constitution. The British system has rested, for generations, on trust. And in the belief that QCs like Grieve would, no matter how angry they are, still not bend the rules in a way that allows a minority of MPs to call the shots in parliament.

It boils down to a simple point: Grieve, and other Tory MPs who last week declared their confidence in the government, do not have sufficient confidence that the government will govern as they wish it to. So they seize - and squirrel away - powers for future use.

Chip away at the foundations of the executive’s power, and it will eventually fall over. And with it, perhaps, the British system of government.