A constitution which is in part based on convention is a remarkable thing, with various advantages. But it is fragile as well as precious.

Our constitution exists in no small part to provide mutual protection from excess or abuse of office – on the part of the executive, but also on the part of any other wielder of power. That includes, of course, Parliament itself, for example in forbidding a current Parliament from binding its successors.

It also extends to the office of Speaker, whose responsibility to champion the rights of Parliament brings with it important powers over legislation and procedure in the Chamber – and who, therefore, must also observe reasonable limits to ensure he is not misusing those powers to serve a personal and political, rather than Parliamentary, agenda.

The fragility of convention is obvious. It rests on a mixture of honour, social expectation, and self-denial in service of a greater cause. It isn’t wholly reliant on pure principle, of course – it’s possible for someone who dislikes the concept of a constitutional protection to still recognise that destroying it would harm their own interests.

The famous exchange between Sir Thomas More and William Roper imagined in A Man For All Seasons illustrates the concept particularly well:

Sadly, it is in the nature of Governments that they tend to push the boundaries, or even flatten conventions outright when given the chance. Administrations of all stripes have done so over the last 20 or more years.

It is an important role of Parliament, and particularly of the Speaker, to try to resist those efforts. So yes, John Bercow has a responsibility to defend the legislature from excesses of the executive – but as a guardian of the constitution he also has an especially important responsibility to obey it himself.

That is why Bercow’s decision today on the Grieve amendment is important and troubling to a degree well in excess of its immediate political impact.

What appears to have happened is as follows.

The Government planned its actions on the basis of clear advice that amendments would not be in order; had the advice been different, it might well have acted differently. The Clerks of the House and the Table Office also acted on the basis of the same view – at least one pro-Brexit MP, Peter Bone, had an amendment rejected on that basis. When Dominic Grieve proposed his amendment, the Clerks advised that it, too, would not be in order. However, the Speaker personally intervened on Grieve’s amendment in particular in order to insist that it be allowed. He does not appear to have instructed the Table Office to allow Bone’s amendment, or notify other MPs that the advice they had been given was now reversed, or sought to publicise that MPs were in fact now able to propose amendments.

This raises several serious constitutional issues.

For a start, a Speaker whom one side of the House already believes to be politically biased has provided another reason for such suspicion. Bercow’s supporters are open about their belief that he is biased. Back when the Cox inquiry into bullying of Parliamentary staff recommended that the Speaker should resign, Margaret Beckett and Emily Thornberry defended him on the grounds that they wanted him to stay for their preferred purposes in the Brexit debate.

Now he has duly intervened, contrary to the expert advice of the Clerks and in breach of their interpretation of Standing Orders, in order to allow a particular amendment that he likes. Other potential amendments were not given an equal privilege. It has long been the view of this site that a biased Speaker, or at least a Speaker whom half the House believes to be biased, cannot effectively do the job – this incident illustrates and underscores that argument.

What’s more, Bercow’s own answers about his decision this afternoon seem to suggest that he had not fully considered the implications of what he was doing before he did it. That is worrying in any Speaker at any time, but downright reckless on such a sensitive issue. Placing one’s personal wishes above the responsibilities of office is bad enough, but doing so on what appears to be such a flimsy basis and brief consideration is even worse.

Constitutionally, who knows what the repercussions will be of a Speaker who selectively disregards the rules as he wishes, or invents new rules as it suits him?

The stability of Parliamentary procedure, effectively the rule of law in the Chamber, is obviously undermined. Neither Government, nor Opposition, nor backbenchers can now be certain that the advice on which they are operating on the assurance of the Clerks will later be honoured by the Chair. No MP can be sure any longer that they and their opponents are going to be working on the basis of equally-applied rules and rights. The outcome of that seems likely, at minimum, to be to encourage all sides to opt for ever more negative Parliamentary tactics, and to further erode what limited trust there is across the Commons – while the Speaker claims he wants more positivity and more trust.

And that’s only in the short term. As James Forsyth fears, today’s events will do lasting – maybe even permanent – harm to the role of Speaker itself:

By shattering convention in this way, Bercow has not just indulged himself, he has given license to future Speakers to do the same – and inspired MPs to permanently fear the prospect that they will do so. In the absence of faith in the true impartiality of the Chair, the obvious alternative is for both parties to ditch the next convention down the list, the respectful alternation of which party provides the Speaker, and instead compete to put in place Speakers whom they openly expect to be their partisan servants.

Even if you like his actions today, or happen to believe the amendment itself to be positive for Parliament, it is important to consider what the permanent cost will be.

Not only would the business of the Commons suffer from a move to more openly partisan, biased Speakers, but Parliament itself will lose out from such a change. A strong Speaker, respected on all sides due to their even-handed pursuit of the role and defence of the rules, is in a better position to defend Parliament’s privileges when they are threatened by the executive. If it becomes acceptable for MPs to simply put in place a Speaker with whom the majority of them agree politically, and to expect that Speaker will use the office to serve their politics, then we risk replacing a bulwark of the legislature with simply another tool of the government of the day – who, of course, command a majority of MPs.

These are all reasons to be concerned – mortified, even – about what has just happened. But the question remains of what can be done about it.

Ultimately, the downside of our constitutional system is that while it can be easy for a dishonourable person to breach, it offers precious few routes to sanction them for doing so. Bercow appears to believe himself to be the House, rather than simply to be charged with serving and protecting it, but if a majority of MPs either like what he has done or think him useful to have in place for future partisan advantage, then he will continue in post.

A Speaker who disregards the rules in a way they like can easily become one who does so in ways they do not like, and Parliament may come to rue the day it allowed its chair to give up its position of cross-party respect. It is deeply unwise, and painfully short-sighted, for MPs to keep Bercow on. But if they were willing to do so after that damning bullying report, and despite all the other reasons he has given to demonstrate his unsuitability for the role, then it seems unlikely they will act this time. Let us hope the next Speaker is able – and willing – to start the long job of undoing the damage.