Labour is sinking into a legal quagmire over whether Jeremy Corbyn should be automatically on the ballot paper now that Angela Eagle, the former shadow Business Secretary, has formally launched her attempt to oust him as party leader by triggering a leadership election.

The big question is whether an incumbent leader needs to win nominations from 50 MPs or MEPs in order to stand in the contest – as a challenger such as Eagle does. Corbyn cannot be sure of clearing this hurdle, as only 40 Labour MPs supported him in a vote of confidence two weeks ago.

Labour’s rulebook is opaque. When Neil Kinnock, the then-leader, was challenged by Tony Benn (with Corbyn at his side) in 1988, Kinnock had to get the requisite number of MP nominations, just as Benn had to. But the rules were rewritten in 2010 and do not say whether a leader who is challenged goes into the ballot or needs any MPs’ backing first.

Inevitably, there are conflicting legal opinions. Labour’s ruling body, its national executive committee (NEC), is due to meet tomorrow to resolve the issue, and the crucial vote could be close.

It shouldn’t be. Whatever you think of Corbyn, it would be outrageous to exclude him from the shortlist. If someone is allowed to stand for his job, then surely he has the right to fight to keep it – morally, whatever the legal position or the political judgement of Labour’s NEC. Whatever his shortcomings, Corbyn deserves a fair fight.

His MP critics are arguing that Corbyn has proved unfit to lead the party in the past nine months, and so has lost the mandate that his MPs gave him a year ago, when he secured 36 nominations (one more than the required 35 -- a lower threshold, as there was a vacancy after Ed Miliband’s resignation).

The Labour members who gave Corbyn an overwhelming 59 per cent mandate in last September’s contest would be understandably livid if their man were excluded from the shortlist. They would rightly see it as a stitch-up by the Labour establishment. If, as Corbyn’s critics claim, he has lost the confidence of members who backed him last year, then the way to test that is in another leadership election – not by allowing Labour MPs to sack him summarily.

People on both sides of Labour’s ever-widening divide insist that they do not want to see a formal split between its socialist and social democratic wings. However, if Labour denies Corbyn the right to fight for his job, it would make a split more likely.

There are some in both camps who now regard a schism as inevitable. There are Labour MPs ready to walk away from the party if Corbyn wins re-election, rather than face a pitched battle in their constituency parties as left-wingers try to deselect them before the next general election. Some might even privately welcome Corbyn's re-election. Equally, some Corbynistas would be ready to turn Momentum, the Corbyn fan club, from a party-in-waiting into a real one.

The awkward moment when Angela Eagle realised journalists had all left her leadership bid launch

Perhaps the row over the Labour rulebook is really the start of a skirmish over which of the two factions becomes the “official” Labour party. Ownership of the Labour brand, the assets, the 200 HQ staff and a valuable core vote of about 20 per cent would be a huge prize. So some Labour MPs might judge that it would be better to kick Corbyn out in the hope that the Corbynistas then walk away, leaving the majority of MPs in the driving seat. But there is no guarantee that the left would walk into such a trap. The Corbynistas might prefer to stay and fight to regain control of the party, confident that a majority of its 500,000-strong membership would be on their side. That would be a recipe for bloody deselection battles, and possibly an attempted left-wing challenge to Eagle before the general election.

The fast-moving events in the Conservative Party could have a knock-on effect on Labour. Although Theresa May has ruled out an early general election, Labour MPs who oppose Corbyn will use her earlier than expected elevation to Downing Street as an argument for dumping him, on the grounds that May could seek her own mandate from the public within months. Corbyn critics claim that he and his operation showed during the EU referendum campaign that they would not withstand the intense pressure and media scrutiny of a general election.

Andrea Leadsom’s surprise withdrawal from the Tory race also makes a realignment of political parties less likely. If she had won and the Tories had lurched right while Labour veered left, it could have created the space for a new, pro-European centre party composed of anti-Corbyn Labourites, the Liberal Democrats and some pro-Cameron Conservatives. Private soundings are being taken by figures in all three parties. There could still be a Lib-Lab link-up after a Labour split but the Tories will now rally strongly behind May.