Chris Williamson, the MP for Derby North, has been readmitted to the Labour Party after serving a four-month suspension for an allegedly anti-Semitic pattern of behaviour.

In the month leading up to the withdrawal of the whip from Williamson at the end of February, he had, among other things: accused of Labour of being "too apologetic" about accusations of anti-Semitism in the party's ranks, attacked critics of an activist suspended for anti-Semitism, and planned a parliamentary screening of a documentary championing the cause of Jackie Walker, another suspended Labour activist.

Incidents such as these - of which there are, frankly, too many to list - have long been a source of consternation not just for Williamson's colleagues in the PLP but for some allies of Jeremy Corbyn too. But his often provocative behaviour when it came to Britain's Jews was for a long time indulged by the leader's office for the simple reason that he was, politically speaking, closer to Corbyn than almost any other Labour backbencher.

When his suspension finally came on 27 February - just over a week after eight Labour MPs quit the party to form the Independent Group - it was taken as a sign of just how committed Corbyn was to preventing any splintering of the PLP over anti-Semitism becoming a full-blown, irrevocable split. There was nothing new or qualitatively different about the behaviour that triggered Williamson's suspension: what had changed was the existence of TIG, which owed its existence first and foremost to the deep disgust many Labour MPs felt, and indeed feel, about the leadership's handling of anti-Semitism cases. (In that light, a letter from 38 Labour MPs demanding Williamson's suspension - the sort of thing the leadership had been empowered to ignore by the 2017 election result - took on an altogether more urgent quality.)

The gravity of the circumstances was as such that Corbyn did something he almost never does, and sacked an ideological ally who had been personally loyal to him after they had already apologised for their behaviour. The moment was an instructive test case as to what the leadership felt could harm the party politically or electorally. Up until then, the only pronouncement of Williamson's that had inspired any sort of movement from Corbyn's office was a freelance suggestion that a Labour government should double council tax bills for the highest-valued homes in January 2018. He was sacked from the frontbench shortly afterwards. But in spite of complaints from Labour MPs and representatives of the Jewish community, the altogether more offensive strain of Williamson's behaviour was tolerated until the political moment changed utterly.

So what does it mean that Williamson, who has been given a formal warning by a disciplinary panel of Labour's ruling national executive committee, is back? The first and most obvious thing to say is that there is considerable anger in the PLP at the decision. Some compare Williamson's treatment to that of Alastair Campbell, auto-excluded from the party for admitting to voting for the Liberal Democrats in the European elections, and make the familiar complaint that, when it comes to disciplinary matters, it is one (flexible) rule for allies of the leader and another for his factional adversaries.

Labour sources, however, point out that the three-person panel that recommended Williamson be let off with a formal warning - a sanction that will be escalated if there is any repeat of his past behaviour - did not have a pro-Corbyn majority. The clear implication is that this was not the leadership's preferred outcome. The decision to overrule a recommendation by party staff that the case be referred to the National Constitutional Committee, Labour's supreme disciplinary body, has been blamed on Williamson's colleague Keith Vaz, who is said to have argued and voted for Williamson's readmission on the grounds that he represents a marginal seat. He was joined by Huda Elmi, elected as part of the #JC9 slate of Corbyn-supporting membership representatives last summer, while George Howarth, who represents the PLP on the NEC, voted against his readmission.

The split lays bare the rift between those who are loyal to Corbyn for reasons of ideology (Elmi) or self-interest (Vaz) and the majority of Labour MPs. None of this will not disabuse a restive PLP of the notion that the party's procedures are unfit for purpose, and that this decision in particular was a wrong and wholly unnecessary call.

Others are more phlegmatic. They believe that the real motivation for today's decision can be found in its timing. Indeed, Vaz went as far to make that argument explicitly, Williamson's readmission means that, like every other Labour MP, he now has 12 days to notify the party of whether he intends to stand at the next general election - the first stage of the inaugural re-selection process to take place under new rules that make it considerably easier for local members to deselect their MP. One of the many grim ironies of Williamson's case is that, despite enthusiastically advocating for open selections for reasons of factional expediency, he is himself at risk from the new rules: Labour sources suggest that several unions intend to trigger a full selection in Derby North.

But that is as much a source of anxiety for Corbynsceptic MPs as it is a relief. Some fear that any attempt to deselect Williamson would likely become a cause célèbre for the left and draw the party into another summer of internecine warfare at the exact moment chances of a general election begin to increase exponentially.