Is this the beginning of the end for Labour MPs? Frank Field, the Labour MP for Birkenhead, has been censured by his constituency Labour party, becoming the second Labour Leaver to face calls for him to be deselected from his local party, after Kate Hoey, the Labour MP for Vauxhall. Is this a sign of things to come? Are Labour MPs worried? And should they be?

What a lot of Labour MPs think is that Hoey and Field are special cases, not just because they are pro-Leave but for a variety of other reasons. Field is, in many ways, as close to a prime candidate for deselection as you can imagine: he doesn’t live in his constituency (though he owns a home there) and some of his parliamentary colleagues say visits rarely is currently at odds with his local party, the area is part of Militant’s old stomping ground, and he is past retirement age. Added to that, his vote was crucial in preventing a government defeat and an early general election and put him on the wrong side of party activist opinion.

As for Hoey, while she has a far greater presence in Vauxhall – even her greatest opponents locally concede she works the area hard – than Field has ever had in Birkenhead, she has been at odds with her local party for more than a decade, first as an outspoken defender of hunting foxes with hounds, then after taking on a part-time role in Boris Johnson’s City Hall. Bailing out the government was merely the latest in a long line of rows.

So if either, and even both, are deselected, Labour MPs will be able to comfort themselves that unlike Field or Hoey, they aren’t at odds with their local parties, they aren’t absentee landlords, and they haven’t just sabotaged an opportunity to trigger an early election. Field and Hoey would have found themselves at odds with their local activists under essentially any of the four other people to run for the Labour leadership in 2015 and 2016.

And all of that will be true, but it comes with a fairly large caveat attached: that under Jeremy Corbyn, unhappy activists are rather more likely to succeed than they once were. Why? Well, it’s necessary to go over the very tricky path to removing a sitting Labour MP.

To deselect a Labour MP they must first be defeated in a trigger ballot: a simple “Yes/No” question in which local party branches vote on whether to endorse the sitting MP. Unless the sitting MP can get 50 per cent of their local branches to back them, a full selection contest is triggered. But here’s where it gets complicated: the number of branches is completely up in the air.

To take Birkenhead as an example, local party members are represented by the six ward branches that make up the constituency. Each branch counts as one vote, regardless of how many party members each branch actually contains. (In the case of Birkenhead, I am told that the Birkenhead and Tranmere ward has the largest number of party members in the constituency, but it will have the exact same weight as any other membership branch.) Adding to the democratic imbalance, affiliated societies and trade unions can affiliate as many branches as they want, each with their own vote, provided they have some members in the constituency in question. And these branches, unlike membership branches, don’t even have to hold a vote – they can be decided by the relevant union official or the head of the local Fabian society at will.

In practice, this gives trades unions a great deal of power over reselections, but it is only really a negative power. If I am the Labour MP for Birkenhead and I am wildly popular with local members, winning all six party branches, yes, the trades union branches can notionally overrule them and force a full selection, but in a full selection, you would assume I’d win the selection easily. But what trades unions can do is prevent unpopular local incumbents having to face the wrath of their local activists in the first place – a very useful tool to guarantee loyalty to the political interests of the trades unions.

In practice, this means that unless you alienate both party members and the party’s power brokers, your chances of not being reselected as a Labour MP are slim absent significant rules changes. And that’s the difference that really matters: Frank Field and Kate Hoey have long been unpopular with their local parties. It’s just that now the party’s power players are just as likely to be agents of deselection as they are to protect the incumbents. And that has repercussions for all Labour MPs.

Update: I originally wrote that Frank Field visits his Birkenhead constituency monthly. Field spends Thursday to Saturday in his constituency every second, fourth and fifh weekend of each month, except in August.